In their arrangement leaves follow a definite order. The points on the stem at which leaves appear are called nodes; the part of the stem between the nodes is theinternode. When two leaves are produced at the same node, one on each side of the stem or axis, and at the same level, they areopposite(fig. 29); when more than two are produced they areverticillate, and the circle of leaves is then called averticilorwhorl. When leaves are opposite, each successive pair may be placed at right angles to the pair immediately preceding. They are then said todecussate, following thus a law of alternation (fig. 29). The same occurs in the verticillate arrangement, the leaves of each whorl rarely beingsuperposedon those of the whorl next it, but usually alternating so that each leaf in a whorl occupies the space between two leaves of the whorl next to it. There are considerable irregularities, however, in this respect, and the number of leaves in different whorls is not always uniform, as may be seen inLysimachia vulgaris. When a single leaf is produced at a node, and the nodes are separated so that each leaf is placed at a different height on the stem, the leaves arealternate(fig. 30). A plane passing through the point of insertion of the leaf in the node, dividing the leaf into similar halves, is the median plane of the leaf; and when the leaves are arranged alternately on an axis so that their median planes coincide they form a straight row ororthostichy. On every axis there are usually two or more orthostichies. In fig. 31, leaf 1 arises from a noden; leaf 2 is separated from it by an internode m, and is placed to the right or left; while leaf 3 is situated directly above leaf 1. In this case, then, there are two orthostichies, and the arrangement is said to bedistichous. When the fourth leaf is directly above the first, the arrangement istristichous. The same arrangement continues throughout the branch, so that in the latter case the 7th leaf is above the 4th, the 10th above the 7th; also the 5th above the 2nd, the 6th above the 3rd and so on. The size of the angle between the median planes of two consecutive leaves in an alternate arrangement is theirdivergence; and it is expressed in fractions of the circumference of the axis which is supposed to be a circle. In a regularly-formed straight branch covered with leaves, if a thread is passed from one to the other, turning always in the same direction, a spiral is described, and a certain number of leaves and of complete turns occur before reaching the leaf directly above that from which the enumeration commenced. If this arrangement is expressed by a fraction, the numerator of which indicates the number of turns, and the denominator the number of internodes in the spiral cycle, the fraction will be found to represent the angle of divergence of the consecutive leaves on the axis. Thus, in fig. 32,a,b, the cycle consists of five leaves, the 6th leaf being placed vertically over the 1st, the 7th over the 2nd and so on; while the number of turns between the 1st and 6th leaf is two; hence this arrangement is indicated by the fraction2⁄5. In other words, the distance or divergence between the first and second leaf, expressed in parts of a circle, is2⁄5of a circle or 360° ×2⁄5= 144°. In fig. 31,a,b, the spiral is ½,i.e.one turn and two leaves; the third leaf being placed vertically over the first, and the divergence between the first and second leaf being one-half the circumference of a circle, 360° × ½ = 180°. Again, in a tristichous arrangement the number is1⁄3, or one turn and three leaves, the angular divergence being 120°.Fig. 31.—Portion of a branch of a Lime tree, with four leaves arranged in a distichous manner, or in two rows.a, The branch with the leaves numbered in their order,nbeing the node andmthe internode;bis a magnified representation of the branch, showing the points of insertion of the leaves and their spiral arrangement, which is expressed by the fraction ½, or one turn of the spiral for two internodes.Fig. 32.—Part of a branch of a Cherry with six leaves, the sixth being placed vertically over the first, after two turns of the spiral. This is expressed by two-fifths.a, The branch, with the leaves numbered in order;b, a magnified representation of the branch, showing the points of insertion of the leaves and their spiral arrangement.By this means we have a convenient mode of expressing on paper the exact position of the leaves upon an axis. And in many cases such a mode of expression is of excellent service in enabling us readily to understand the relations of the leaves. The divergences may also be represented diagrammatically on a horizontal projection of the vertical axis, as in fig. 33. Here the outermost circle represents a section of that portion of the axis bearing the lowest leaf, the innermost represents the highest. The broad dark lines represent the leaves, and they are numbered according to their age and position. It will be seen at once that the leaves are arranged in orthostichies marked I.-V., and that these divide the circumference into five equal portions. But the divergence between leaf 1 and leaf 2 is equal to2⁄5ths of the circumference, and the same is the case between 2 and 3, 3 and 4, &c. The divergence, then, is2⁄5, and from this we learn that, starting from any leaf on the axis, we must pass twice round the stem in a spiral through five leaves before reaching one directly over that with which we started. The line which, winding round an axis either to the right or to the left, passes through the points of insertion of all the leaves on the axis is termed thegeneticorgenerating spiral; and that margin of each leaf which is towards the direction from which the spiral proceeds is thekathodicside, the other margin facing the point whither the spiral passes being theanodicside.In cases where the internodes are very short and the leaves are closely applied to each other, as in the house-leek, it is difficult to trace thegenerating spiral. Thus, in fig. 34 there are thirteen leaves which are numbered in their order, and five turns of the spiral marked by circles in the centre (5⁄13indicating the arrangement); but this could not be detected at once. So also in fir cones (fig. 35), which are composed of scales or modified leaves, the generating spiral cannot be determined easily. But in such cases a series ofsecondary spiralsorparastichiesare seen running parallel with each other both right and left, which to a certain extent conceal the genetic spiral.Fig. 33.—Diagram of a phyllotaxis represented by the fraction2⁄5.The spiral is not always constant throughout the whole length of an axis. The angle of divergence may alter either abruptly or gradually, and the phyllotaxis thus becomes very complicated. This change may be brought about by arrest of development, by increased development of parts or by a torsion of the axis. The former are exemplified in many Crassulaceae and aloes. The latter is seen well in the screw-pine (Pandanus). In the bud of the screw-pine the leaves are arranged in three orthostichies with the phyllotaxis1⁄3, but by torsion the developed leaves become arranged in three strong spiral rows running round the stem. These causes of change in phyllotaxis are also well exemplified in the alteration of an opposite or verticillate arrangement to an alternate, and vice versa; thus the effect of interruption of growth, in causing alternate leaves to become opposite and verticillate, can be distinctly shown inRhododendron ponticum. The primitive or generating spiral maypass either from right to left or from left to right. It sometimes follows a different direction in the branches from that pursued in the stem. When it follows the same course in the stem and branches, they arehomodromous; when the direction differs, they areheterodromous. In different species of the same genus the phyllotaxis frequently varies.All modifications of leaves follow the same laws of arrangement as true leaves—a fact which is of importance in a morphological point of view. In dicotyledonous plants the first leaves produced (the cotyledons) are opposite. This arrangement often continues during the life of the plant, but at other times it changes, passing into distichous and spiral forms. Some tribes of plants are distinguished by their opposite or verticillate, others by their alternate, leaves. Labiate plants have decussate leaves, while Boraginaceae have alternate leaves, and Tiliaceae usually have distichous leaves; Rubiaceae have opposite leaves. Such arrangements as2⁄5,3⁄8,5⁄13and8⁄21are common in Dicotyledons. The first of these, called aquincunx, is met with in the apple, pear and cherry (fig. 32); the second, in the bay, holly,Plantago media; the third, in the cones ofPicea alba(fig. 35); and the fourth in those of the silver fir. In monocotyledonous plants there is only one seed-leaf or cotyledon, and hence the arrangement is at first alternate; and it generally continues so more or less, rarely being verticillate. Such arrangements as1⁄2,1⁄3and2⁄3are common in Monocotyledons, as in grasses, sedges and lilies. It has been found in general that, while the number 5 occurs in the phyllotaxis of Dicotyledons, 3 is common in that of Monocotyledons.Fig. 34.—Cycle of thirteen leaves placed closely together so as to form a rosette, as inSempervivum. A is the very short axis to which the leaves are attached. The leaves are numbered in their order, from below upwards. The circles in the centre indicate the five turns of the spiral, and show the insertion of each of the leaves. The divergence is expressed by the fraction5⁄13ths.Fig. 35.—Cone ofPicea albawith the scales or modified leaves numbered in the order of their arrangement on the axis of the cone. The lines indicate a rectilinear series of scales and two lateral secondary spirals, one turning from left to right, the other from right to left.In the axil of previously formed leaves leaf-buds arise. These leaf-buds contain the rudiments of a shoot, and consist of leaves covering a growing point. The buds of trees of temperate climates, which lie dormant during the winter, are protected by scale leaves. These scales or protective appendages of the bud consist either of the altered laminae or of the enlarged petiolary sheath, or of stipules, as in the fig and magnolia, or of one or two of these parts combined. These are often of a coarse nature, serving a temporary purpose, and then falling off when the leaf is expanded. They are frequently covered with a resinous matter, as in balsam-poplar and horse-chestnut, or by a thick downy covering as in the willow. In plants of warm climates the buds have often no protective appendages, and are then said to benaked.Fig. 36.Fig. 37.Fig. 38.Fig. 39.Fig. 40.Fig. 41.Fig. 36.—Circinate vernation.Fig. 37.—Transverse section of a conduplicate leaf.Fig. 38.—Transverse section of a plicate or plaited leaf.Fig. 39.—Transverse section of a convolute leaf.Fig. 40.—Transverse section of an involute leaf.Fig. 41.—Transverse section of a revolute leaf.Fig. 42.Fig. 43.Fig. 44.Fig. 45.Fig. 42.—Transverse section of a bud, in which the leaves are arranged in an accumbent manner.Fig. 43.—Transverse section of a bud, in which the leaves are arranged in an equitant manner.Fig. 44.—Transverse section of a bud, showing two leaves folded in an obvolute manner. Each is conduplicate, and one embraces the edge of the other.Fig. 45.—Transverse section of a bud, showing two leaves arranged in a supervolute manner.The arrangement of the leaves in the bud is termedvernationorprefoliation. In considering vernation we must take into account both the manner in which each individual leaf is folded and also the arrangement of the leaves in relation to each other. These vary in different plants, but in each species they follow a regular law. The leaves in the bud are either placed simply in apposition, as in the mistletoe, or they are folded or rolled up longitudinally or laterally, giving rise to different kinds of vernation, as delineated in figs. 36 to 45, where the folded or curved lines represent the leaves, the thickened part being the midrib. The leaf taken individually is either folded longitudinally from apex to base, as in the tulip-tree, and calledreclinateorreplicate; or rolled up in a circular manner from apex to base, as in ferns (fig. 36), and calledcircinate; or folded laterally,conduplicate(fig. 37), as in oak; or it has several folds like a fan,plicateorplaited(fig. 38), as in vine and sycamore, and in leaves with radiating vernation, where the ribs mark the foldings; or it is rolled upon itself,convolute(fig. 39), as in banana and apricot; or its edges are rolled inwards,involute(fig. 40), as in violet; or outwards,revolute(fig. 41), as in rosemary. The different divisions of a cut leaf may be folded or rolled up separately, as in ferns, while the entire leaf may have either the same or a different kind of vernation. The leaves have a definite relation to each other in the bud, being either opposite, alternate or verticillate; and thus different kinds of vernation are produced. Sometimes they are nearly in a circle at the same level, remaining flat or only slightly convex externally, and placed so as to touch each other by their edges, thus giving rise tovalvatevernation. At other times they are at different levels, and are applied over each other, so as to beimbricated, as in lilac, and in the outer scales of sycamore; and occasionally the margin of one leaf overlaps that of another, while it in its turn is overlapped by a third, so as to betwisted,spiralorcontortive. When leaves are applied to each other face to face, without being folded or rolled together, they areappressed. When the leaves are more completely folded they either touch at their extremities and areaccumbentoropposite(fig. 42), or are folded inwards by their margin and becomeinduplicate; or a conduplicate leaf covers another similarly folded, which in turn covers a third, and thus the vernation isequitant(fig. 43), as in privet; or conduplicate leaves are placed so that the half of the one covers the half of another, and thus they becomehalf-equitantorobvolute(fig. 44), as in sage. When in the case of convolute leaves one leaf is rolled up within the other, it issupervolute(fig. 45). The scales of a bud sometimes exhibit one kind of vernation and the leaves another. The same modes of arrangement occur in the flower-buds.Leaves, after performing their functions for a certain time, wither and die. In doing so they frequently change colour, and hence arise the beautiful and varied tints of the autumnal foliage. This changeof colour is chiefly occasioned by the diminished circulation in the leaves, and the higher degree of oxidation to which their chlorophyll has been submitted.Leaves which are articulated with the stem, as in the walnut and horse-chestnut, fall and leave a scar, while those which are continuous with it remain attached for some time after they have lost their vitality. Most of the trees of Great Britain have deciduous leaves, their duration not extending over more than a few months, while in trees of warm climates the leaves often remain for two or more years. In tropical countries, however, many trees lose their leaves in the dry season. The period of defoliation varies in different countries according to the nature of their climate. Trees which are called evergreen, as pines and evergreen-oak, are always deprived of a certain number of leaves at intervals, sufficient being left, however, to preserve their green appearance. The cause of the fall of the leaf in cold climates seems to be deficiency of light and heat in winter, which causes a cessation in the functions of the cells of the leaf. The fall is directly caused by the formation of a layer of tissue across the base of the leaf-stalk; the cells of this layer separate from one another and the leaf remains attached only by the fibres of the veins until it becomes finally detached by the wind or frost. Before its fall the leaf has become dry owing to loss of water and the removal of the protoplasm and food substances to the stem for use next season; the red and yellow colouring matters are products of decomposition of the chlorophyll. Inorganic and other waste matters are stored in the leaf-tissue and thus got rid of by the plant. The leaf scar is protected by a corky change (suberization) in the walls of the exposed cells.
In their arrangement leaves follow a definite order. The points on the stem at which leaves appear are called nodes; the part of the stem between the nodes is theinternode. When two leaves are produced at the same node, one on each side of the stem or axis, and at the same level, they areopposite(fig. 29); when more than two are produced they areverticillate, and the circle of leaves is then called averticilorwhorl. When leaves are opposite, each successive pair may be placed at right angles to the pair immediately preceding. They are then said todecussate, following thus a law of alternation (fig. 29). The same occurs in the verticillate arrangement, the leaves of each whorl rarely beingsuperposedon those of the whorl next it, but usually alternating so that each leaf in a whorl occupies the space between two leaves of the whorl next to it. There are considerable irregularities, however, in this respect, and the number of leaves in different whorls is not always uniform, as may be seen inLysimachia vulgaris. When a single leaf is produced at a node, and the nodes are separated so that each leaf is placed at a different height on the stem, the leaves arealternate(fig. 30). A plane passing through the point of insertion of the leaf in the node, dividing the leaf into similar halves, is the median plane of the leaf; and when the leaves are arranged alternately on an axis so that their median planes coincide they form a straight row ororthostichy. On every axis there are usually two or more orthostichies. In fig. 31, leaf 1 arises from a noden; leaf 2 is separated from it by an internode m, and is placed to the right or left; while leaf 3 is situated directly above leaf 1. In this case, then, there are two orthostichies, and the arrangement is said to bedistichous. When the fourth leaf is directly above the first, the arrangement istristichous. The same arrangement continues throughout the branch, so that in the latter case the 7th leaf is above the 4th, the 10th above the 7th; also the 5th above the 2nd, the 6th above the 3rd and so on. The size of the angle between the median planes of two consecutive leaves in an alternate arrangement is theirdivergence; and it is expressed in fractions of the circumference of the axis which is supposed to be a circle. In a regularly-formed straight branch covered with leaves, if a thread is passed from one to the other, turning always in the same direction, a spiral is described, and a certain number of leaves and of complete turns occur before reaching the leaf directly above that from which the enumeration commenced. If this arrangement is expressed by a fraction, the numerator of which indicates the number of turns, and the denominator the number of internodes in the spiral cycle, the fraction will be found to represent the angle of divergence of the consecutive leaves on the axis. Thus, in fig. 32,a,b, the cycle consists of five leaves, the 6th leaf being placed vertically over the 1st, the 7th over the 2nd and so on; while the number of turns between the 1st and 6th leaf is two; hence this arrangement is indicated by the fraction2⁄5. In other words, the distance or divergence between the first and second leaf, expressed in parts of a circle, is2⁄5of a circle or 360° ×2⁄5= 144°. In fig. 31,a,b, the spiral is ½,i.e.one turn and two leaves; the third leaf being placed vertically over the first, and the divergence between the first and second leaf being one-half the circumference of a circle, 360° × ½ = 180°. Again, in a tristichous arrangement the number is1⁄3, or one turn and three leaves, the angular divergence being 120°.
By this means we have a convenient mode of expressing on paper the exact position of the leaves upon an axis. And in many cases such a mode of expression is of excellent service in enabling us readily to understand the relations of the leaves. The divergences may also be represented diagrammatically on a horizontal projection of the vertical axis, as in fig. 33. Here the outermost circle represents a section of that portion of the axis bearing the lowest leaf, the innermost represents the highest. The broad dark lines represent the leaves, and they are numbered according to their age and position. It will be seen at once that the leaves are arranged in orthostichies marked I.-V., and that these divide the circumference into five equal portions. But the divergence between leaf 1 and leaf 2 is equal to2⁄5ths of the circumference, and the same is the case between 2 and 3, 3 and 4, &c. The divergence, then, is2⁄5, and from this we learn that, starting from any leaf on the axis, we must pass twice round the stem in a spiral through five leaves before reaching one directly over that with which we started. The line which, winding round an axis either to the right or to the left, passes through the points of insertion of all the leaves on the axis is termed thegeneticorgenerating spiral; and that margin of each leaf which is towards the direction from which the spiral proceeds is thekathodicside, the other margin facing the point whither the spiral passes being theanodicside.
In cases where the internodes are very short and the leaves are closely applied to each other, as in the house-leek, it is difficult to trace thegenerating spiral. Thus, in fig. 34 there are thirteen leaves which are numbered in their order, and five turns of the spiral marked by circles in the centre (5⁄13indicating the arrangement); but this could not be detected at once. So also in fir cones (fig. 35), which are composed of scales or modified leaves, the generating spiral cannot be determined easily. But in such cases a series ofsecondary spiralsorparastichiesare seen running parallel with each other both right and left, which to a certain extent conceal the genetic spiral.
The spiral is not always constant throughout the whole length of an axis. The angle of divergence may alter either abruptly or gradually, and the phyllotaxis thus becomes very complicated. This change may be brought about by arrest of development, by increased development of parts or by a torsion of the axis. The former are exemplified in many Crassulaceae and aloes. The latter is seen well in the screw-pine (Pandanus). In the bud of the screw-pine the leaves are arranged in three orthostichies with the phyllotaxis1⁄3, but by torsion the developed leaves become arranged in three strong spiral rows running round the stem. These causes of change in phyllotaxis are also well exemplified in the alteration of an opposite or verticillate arrangement to an alternate, and vice versa; thus the effect of interruption of growth, in causing alternate leaves to become opposite and verticillate, can be distinctly shown inRhododendron ponticum. The primitive or generating spiral maypass either from right to left or from left to right. It sometimes follows a different direction in the branches from that pursued in the stem. When it follows the same course in the stem and branches, they arehomodromous; when the direction differs, they areheterodromous. In different species of the same genus the phyllotaxis frequently varies.
All modifications of leaves follow the same laws of arrangement as true leaves—a fact which is of importance in a morphological point of view. In dicotyledonous plants the first leaves produced (the cotyledons) are opposite. This arrangement often continues during the life of the plant, but at other times it changes, passing into distichous and spiral forms. Some tribes of plants are distinguished by their opposite or verticillate, others by their alternate, leaves. Labiate plants have decussate leaves, while Boraginaceae have alternate leaves, and Tiliaceae usually have distichous leaves; Rubiaceae have opposite leaves. Such arrangements as2⁄5,3⁄8,5⁄13and8⁄21are common in Dicotyledons. The first of these, called aquincunx, is met with in the apple, pear and cherry (fig. 32); the second, in the bay, holly,Plantago media; the third, in the cones ofPicea alba(fig. 35); and the fourth in those of the silver fir. In monocotyledonous plants there is only one seed-leaf or cotyledon, and hence the arrangement is at first alternate; and it generally continues so more or less, rarely being verticillate. Such arrangements as1⁄2,1⁄3and2⁄3are common in Monocotyledons, as in grasses, sedges and lilies. It has been found in general that, while the number 5 occurs in the phyllotaxis of Dicotyledons, 3 is common in that of Monocotyledons.
In the axil of previously formed leaves leaf-buds arise. These leaf-buds contain the rudiments of a shoot, and consist of leaves covering a growing point. The buds of trees of temperate climates, which lie dormant during the winter, are protected by scale leaves. These scales or protective appendages of the bud consist either of the altered laminae or of the enlarged petiolary sheath, or of stipules, as in the fig and magnolia, or of one or two of these parts combined. These are often of a coarse nature, serving a temporary purpose, and then falling off when the leaf is expanded. They are frequently covered with a resinous matter, as in balsam-poplar and horse-chestnut, or by a thick downy covering as in the willow. In plants of warm climates the buds have often no protective appendages, and are then said to benaked.
Fig. 36.—Circinate vernation.Fig. 37.—Transverse section of a conduplicate leaf.Fig. 38.—Transverse section of a plicate or plaited leaf.Fig. 39.—Transverse section of a convolute leaf.Fig. 40.—Transverse section of an involute leaf.Fig. 41.—Transverse section of a revolute leaf.
Fig. 36.—Circinate vernation.
Fig. 37.—Transverse section of a conduplicate leaf.
Fig. 38.—Transverse section of a plicate or plaited leaf.
Fig. 39.—Transverse section of a convolute leaf.
Fig. 40.—Transverse section of an involute leaf.
Fig. 41.—Transverse section of a revolute leaf.
Fig. 42.—Transverse section of a bud, in which the leaves are arranged in an accumbent manner.
Fig. 43.—Transverse section of a bud, in which the leaves are arranged in an equitant manner.
Fig. 44.—Transverse section of a bud, showing two leaves folded in an obvolute manner. Each is conduplicate, and one embraces the edge of the other.
Fig. 45.—Transverse section of a bud, showing two leaves arranged in a supervolute manner.
The arrangement of the leaves in the bud is termedvernationorprefoliation. In considering vernation we must take into account both the manner in which each individual leaf is folded and also the arrangement of the leaves in relation to each other. These vary in different plants, but in each species they follow a regular law. The leaves in the bud are either placed simply in apposition, as in the mistletoe, or they are folded or rolled up longitudinally or laterally, giving rise to different kinds of vernation, as delineated in figs. 36 to 45, where the folded or curved lines represent the leaves, the thickened part being the midrib. The leaf taken individually is either folded longitudinally from apex to base, as in the tulip-tree, and calledreclinateorreplicate; or rolled up in a circular manner from apex to base, as in ferns (fig. 36), and calledcircinate; or folded laterally,conduplicate(fig. 37), as in oak; or it has several folds like a fan,plicateorplaited(fig. 38), as in vine and sycamore, and in leaves with radiating vernation, where the ribs mark the foldings; or it is rolled upon itself,convolute(fig. 39), as in banana and apricot; or its edges are rolled inwards,involute(fig. 40), as in violet; or outwards,revolute(fig. 41), as in rosemary. The different divisions of a cut leaf may be folded or rolled up separately, as in ferns, while the entire leaf may have either the same or a different kind of vernation. The leaves have a definite relation to each other in the bud, being either opposite, alternate or verticillate; and thus different kinds of vernation are produced. Sometimes they are nearly in a circle at the same level, remaining flat or only slightly convex externally, and placed so as to touch each other by their edges, thus giving rise tovalvatevernation. At other times they are at different levels, and are applied over each other, so as to beimbricated, as in lilac, and in the outer scales of sycamore; and occasionally the margin of one leaf overlaps that of another, while it in its turn is overlapped by a third, so as to betwisted,spiralorcontortive. When leaves are applied to each other face to face, without being folded or rolled together, they areappressed. When the leaves are more completely folded they either touch at their extremities and areaccumbentoropposite(fig. 42), or are folded inwards by their margin and becomeinduplicate; or a conduplicate leaf covers another similarly folded, which in turn covers a third, and thus the vernation isequitant(fig. 43), as in privet; or conduplicate leaves are placed so that the half of the one covers the half of another, and thus they becomehalf-equitantorobvolute(fig. 44), as in sage. When in the case of convolute leaves one leaf is rolled up within the other, it issupervolute(fig. 45). The scales of a bud sometimes exhibit one kind of vernation and the leaves another. The same modes of arrangement occur in the flower-buds.
Leaves, after performing their functions for a certain time, wither and die. In doing so they frequently change colour, and hence arise the beautiful and varied tints of the autumnal foliage. This changeof colour is chiefly occasioned by the diminished circulation in the leaves, and the higher degree of oxidation to which their chlorophyll has been submitted.
Leaves which are articulated with the stem, as in the walnut and horse-chestnut, fall and leave a scar, while those which are continuous with it remain attached for some time after they have lost their vitality. Most of the trees of Great Britain have deciduous leaves, their duration not extending over more than a few months, while in trees of warm climates the leaves often remain for two or more years. In tropical countries, however, many trees lose their leaves in the dry season. The period of defoliation varies in different countries according to the nature of their climate. Trees which are called evergreen, as pines and evergreen-oak, are always deprived of a certain number of leaves at intervals, sufficient being left, however, to preserve their green appearance. The cause of the fall of the leaf in cold climates seems to be deficiency of light and heat in winter, which causes a cessation in the functions of the cells of the leaf. The fall is directly caused by the formation of a layer of tissue across the base of the leaf-stalk; the cells of this layer separate from one another and the leaf remains attached only by the fibres of the veins until it becomes finally detached by the wind or frost. Before its fall the leaf has become dry owing to loss of water and the removal of the protoplasm and food substances to the stem for use next season; the red and yellow colouring matters are products of decomposition of the chlorophyll. Inorganic and other waste matters are stored in the leaf-tissue and thus got rid of by the plant. The leaf scar is protected by a corky change (suberization) in the walls of the exposed cells.
(A. B. R.)
LEAF-INSECT,the name given to orthopterous insects of the family Phasmidae, referred to the single genusPhylliumand characterized by the presence of lateral laminae upon the legs and abdomen, which, in association with an abundance of green colouring-matter, impart a broad and leaf-like appearance to the whole insect. In the female this deceptive resemblance is enhanced by the large size and foliaceous form of the front wings which, when at rest edge to edge on the abdomen, forcibly suggest in their neuration the midrib and costae of an ordinary leaf. In this sex the posterior wings are reduced and functionless so far as flight is concerned; in the male they are ample, membranous and functional, while the anterior wings are small and not leaf-like. The freshly hatched young are reddish in colour; but turn green after feeding for a short time upon leaves. Before death a specimen has been observed to pass through the various hues of a decaying leaf, and the spectrum of the green colouring matter does not differ from that of the chlorophyll of living leaves. Since leaf-insects are purely vegetable feeders and not predaceous like mantids, it is probable that their resemblance to leaves is solely for purposes of concealment from enemies. Their egg capsules are similarly protected by their likeness to various seeds. Leaf-insects range from India to the Seychelles on the one side, and to the Fiji Islands on the other.
(R. I. P.)
LEAGUE. 1. (Through Fr.ligue, Ital.liga, from Lat.ligare, to bind), an agreement entered into by two or more parties for mutual protection or joint attack, or for the furtherance of some common object, also the body thus joined or “leagued” together. The name has been given to numerous confederations, such as the Achaean League (q.v.), the confederation of the ancient cities of Achaia, and especially to the various holy leagues (ligues saintes), of which the better known are those formed by Pope Julius II. against Venice in 1508, often known as the League of Cambrai, and against France in 1511. “The League,” in French history, is that of the Catholics headed by the Guises to preserve the Catholic religion against the Huguenots and prevent the accession of Henry of Navarre to the throne (seeFrance:History). “The Solemn League and Covenant” was the agreement for the establishment of Presbyterianism in both countries entered into by England and Scotland in 1643 (seeCovenanters). Of commercial leagues the most famous is that of the Hanse towns, known as the Hanseatic League (q.v.). The word has been adopted by political associations, such as the Anti-Corn Law League, the Irish Land League, the Primrose League and the United Irish League, and by numerous social organizations. “League” has also been applied to a special form of competition in athletics, especially in Association football. In this system clubs “league” together in a competition, each playing every other member of the association twice, and the order of merit is decided by the points gained during the season, a win counting two and a draw one.
2. (From the late Lat.leuga, orleuca, said to be a Gallic word; the mod. Fr.lieuecomes from the O. Fr.liue; the Gaelicleac, meaning a flat stone posted as a mark of distance on a road, has been suggested as the origin), a measure of distance, probably never in regular use in England, and now only in poetical or rhetorical language. It was the Celtic as opposed to the Teutonic unit, and was used in France, Spain, Portugal and Italy. In all the countries it varies with different localities, and the ancient distance has never been fixed. The kilometric league of France is fixed at four kilometres. The nautical league is equal to three nautical miles.
LEAKE, WILLIAM MARTIN(1777-1860), British antiquarian and topographer, was born in London on the 14th of January 1777. After completing his education at the Royal Military Academy, Woolwich, and spending four years in the West Indies as lieutenant of marine artillery, he was sent by the government to Constantinople to instruct the Turks in this branch of the service. A journey through Asia Minor in 1800 to join the British fleet at Cyprus inspired him with an interest in antiquarian topography. In 1801, after travelling across the desert with the Turkish army to Egypt, he was, on the expulsion of the French, employed in surveying the valley of the Nile as far as the cataracts; but having sailed with the ship engaged to convey the Elgin marbles from Athens to England, he lost all his maps and observations when the vessel foundered off Cerigo. Shortly after his arrival in England he was sent out to survey the coast of Albania and the Morea, with the view of assisting the Turks against attacks of the French from Italy, and of this he took advantage to form a valuable collection of coins and inscriptions and to explore ancient sites. In 1807, war having broken out between Turkey and England, he was made prisoner at Salonica; but, obtaining his release the same year, he was sent on a diplomatic mission to Ali Pasha of Iannina, whose confidence he completely won, and with whom he remained for more than a year as British representative. In 1810 he was granted a yearly sum of £600 for his services in Turkey. In 1815 he retired from the army, in which he held the rank of colonel, devoting the remainder of his life to topographical and antiquarian studies, the results of which were given to the world in the following volumes:Topography of Athens(1821);Journal of a Tour in Asia Minor(1824);Travels in the Morea(1830), and a supplement,Peloponnesiaca(1846);Travels in Northern Greece(1835); andNumismata Hellenica(1854), followed by a supplement in 1859. A characteristic of the researches of Leake was their comprehensive minuteness, which was greatly aided by his mastery of technical details. HisTopography of Athens, the first attempt at a scientific treatment of the subject, is still authoritative in regard to many important points (seeAthens). He died at Brighton on the 6th of January 1860. The marbles collected by him in Greece were presented to the British Museum; his bronzes, vases, gems and coins were purchased by the university of Cambridge after his death, and are now in the Fitzwilliam Museum. He was elected F.R.S. and F.R.G.S., received the honorary D.C.L. at Oxford (1816), and was a member of the Berlin Academy of Sciences and correspondent of the Institute of France.
SeeMemoirby J. H. Marsden (1864); theArchitectfor the 7th of October 1876; E. Curtius in thePreussische Jahrbücher(Sept., 1876); J. E. Sandys,Hist. of Classical Scholarship, iii. (1908), p. 442.
SeeMemoirby J. H. Marsden (1864); theArchitectfor the 7th of October 1876; E. Curtius in thePreussische Jahrbücher(Sept., 1876); J. E. Sandys,Hist. of Classical Scholarship, iii. (1908), p. 442.
LEAMINGTON,a municipal borough and health resort of Warwickshire, England, on the river Leam near its junction with the Avon, 98 m. N.W. from London, served by the Great Western and London & North Western railways. Pop. (1901) 26,888. The parliamentary boroughs of Leamington and Warwick were joined into one constituency in 1885, returning one member. The centres of the towns are 2 m. apart, Warwick lying to the west, but they are united by the intermediate parish of New Milverton. There are three saline springs, and the principal pump-rooms, baths and pleasant gardens lie on the right bank of the river. The chief publicbuildings are the town hall (1884), containing a free library and school of art; and the Theatre Royal and assembly room. The parish church of All Saints is modernized, and the other churches are entirely modern. The S. Warwickshire hospital and Midland Counties Home for incurables are here. Leamington High School is an important school for girls. There is a municipal technical school. Industries include iron foundries and brickworks. The town lies in a well-wooded and picturesque country, within a few miles of such interesting towns as Warwick, Kenilworth, Coventry and Stratford-on-Avon. It is a favourite hunting centre, and, as a health resort, attracts not only visitors but residents. The town is governed by a mayor, 8 aldermen, and 24 councillors. Area, 2817 acres.
Leamington was a village of no importance until about 1786, when baths were first erected, though the springs were noticed by Camden, writing about 1586. The population in 1811 was only 543, The town was incorporated in 1875. The name in former use was Leamington Priors, in distinction from Leamington Hastings, a village on the upper Leam. By royal licence granted in 1838 it was called Royal Leamington Spa.
Leamington was a village of no importance until about 1786, when baths were first erected, though the springs were noticed by Camden, writing about 1586. The population in 1811 was only 543, The town was incorporated in 1875. The name in former use was Leamington Priors, in distinction from Leamington Hastings, a village on the upper Leam. By royal licence granted in 1838 it was called Royal Leamington Spa.
LÉANDRE, CHARLES LUCIEN(1862- ), French caricaturist and painter, was born at Champsecret (Orne), and studied painting under Bin and Cabanel. From 1887 he figured among the exhibitors of the Salon, where he showed numerous portraits and genre pictures, but his popular fame is due to his comic drawings and caricatures. The series of the “Gotha des souverains,” published inLe Rire, placed him in the front rank of modern caricaturists. Besides his contributions toLe Rire,Le Figaroand other comic journals, he published a series of albums:Nocturnes,Le Musée des souverains, andParis et la province. Léandre produced admirable work in lithography, and designed many memorable posters, such as the “Yvette Guilbert.” “Les nouveaux mariés,” “Joseph Prudhomme,” “Les Lutteurs,” and “La Femme au chien.” He was created a knight of the Legion of Honour.
LEAP-YEAR(more properly known asbissextile), the name given to the year containing 366 days. The astronomers of Julius Caesar, 46B.C., settled the solar year at 365 days 6 hours. These hours were set aside and at the end of four years made a day which was added to the fourth year. The English name for the bissextile year is an allusion to the result of the interposition of the extra day; for after the 29th of February a date “leaps over” the day of the week on which it would fall in ordinary years. Thus a birthday on the 10th of June, a Monday, will in the next year, if a leap-year, be on the 10th of June, a Wednesday. Of the origin of the custom for women to woo, not be wooed, during leap-year no satisfactory explanation has ever been offered. In 1288 a law was enacted in Scotland that “it is statut and ordaint that during the rein of hir maist blissit Megeste, for ilk yeare knowne as lepe yeare, ilk mayden ladye of bothe highe and lowe estait shall hae liberte to bespeke ye man she likes, albeit he refuses to taik hir to be his lawful wyfe, he shall be mulcted in ye sum ane pundis or less, as his estait may be; except and awis gif he can make it appeare that he is betrothit ane ither woman he then shall be free.” A few years later a like law was passed in France, and in the 15th century the custom was legalized in Genoa and Florence.
LEAR, EDWARD(1812-1888), English artist and humorist, was born in London on the 12th of May 1812. His earliest drawings were ornithological. When he was twenty years old he published a brilliantly coloured selection of the rarer Psittacidae. Its power attracted the attention of the 13th earl of Derby, who employed Lear to draw his Knowsley menagerie. He became a permanent favourite with the Stanley family; and Edward, 15th earl, was the child for whose amusement the firstBook of Nonsensewas composed. From birds Lear turned to landscape, his earlier efforts in which recall the manner of J. D. Harding; but he quickly acquired a more individual style. About 1837 he set up a studio at Rome, where he lived for ten years, with summer tours in Italy and Sicily, and occasional visits to England. During this period he began to publish hisIllustrated Journals of a Landscape Painter: charmingly written reminiscences of wandering, which ultimately embraced Calabria, the Abruzzi, Albania, Corsica, &c. From 1848-1849 he explored Greece, Constantinople, the Ionian Islands, Lower Egypt, the wildest recesses of Albania, and the desert of Sinai. He returned to London, but the climate did not suit him. In 1854-1855 he wintered on the Nile, and migrated successively to Corfu, Malta and Rome, finally building himself a villa at San Remo. From Corfu Lear visited Mount Athos, Syria, Palestine, and Petra; and when over sixty, by the assistance of Lord Northbrock, then Govenor-General, he saw the cities and scenery of greatest interest within a large area of India. From first to last he was, in whatever circumstances of difficulty or ill-health, an indomitable traveller. Before visiting new lands he studied their geography and literature, and then went straight for the mark; and wherever he went he drew most indefatigably and most accurately. His sketches are not only the basis of more finished works, but an exhaustive record in themselves. Some defect of technique or eyesight occasionally left his larger oil painting, though nobly conceived, crude or deficient in harmony; but his smaller pictures and more elaborate sketches abound in beauty, delicacy, and truth. Lear modestly called himself a topographical artist; but he included in the term the perfect rendering of all characteristic graces of form, colour, and atmosphere. The last task he set himself was to prepare for popular circulation a set of some 200 drawings, illustrating from his travels the scenic touches of Tennyson’s poetry; but he did not live to complete the scheme, dying at San Remo on the 30th of January 1888. Until sobered by age, his conversation was brimful of humorous fun. The paradoxical originality and ostentatiously uneducated draughtsmanship of his numerous nonsense books won him a more universal fame than his serious work. He had a true artist’s sympathy with art under all forms, and might have become a skilled musician had he not been a painter. Swainson, the naturalist, praised young Lear’s great red and yellow macaw as “equalling any figure ever painted by Audubon in grace of design, perspective, and anatomical accuracy.” Murchison, examining his sketches, complimented them as rigorously embodying geological truth. Tennyson’s lines “To E.L. on his Travels in Greece,” mark the poet’s genuine admiration of a cognate spirit in classical art. Ruskin placed theBook of Nonsensefirst in the list of a hundred delectable volumes of contemporary literature, a judgment endorsed by English-speaking children all over the world.
SeeLetters of Edward Lear to Chichester Fortescue, Lord Carlingford, and Frances, Countess Waldegrave(1907), edited by Lady Strachey, with an introduction by Henry Strachey.
SeeLetters of Edward Lear to Chichester Fortescue, Lord Carlingford, and Frances, Countess Waldegrave(1907), edited by Lady Strachey, with an introduction by Henry Strachey.
(F. L.*)
LEASE(derived through the Fr. from the Lat.laxare, to loosen), a certain form of tenure, or the contract embodying it, of land, houses, &c.; seeLandlord and Tenant.
LEATHER(a word which appears in all Teutonic languages; cf. Ger.Leder, Dutchleerorleder, Swed.läder, and in such Celtic forms as Welshllader), an imputrescible substance prepared from the hides or skins of living creatures, both cold and warm blooded, by chemical and mechanical treatment. Skins in the raw and natural moist state are readily putrescible, and are easily disintegrated by bacterial or chemical action, and if dried in this condition become harsh, horny and intractable. The art of the leather manufacturer is principally directed to overcoming the tendency to putrefaction, securing suppleness in the material, rendering it impervious to and unalterable by water, and increasing the strength of the skin and its power to resist wear and tear.
Leather is made by three processes or with three classes of substances. Thus we have (1) tanned leather, in which the hides and skins are combined with tannin or tannic acid; (2) tawed leather, in which the skins are prepared with mineral salts; (3) chamoised (shamoyed) leather, in which the skins are rendered imputrescible by treatment with oils and fats, the decomposition products of which are the actual tanning agents.
Sources and Qualities of Hides and Skins.—The hides used in heavy leather manufacture may be divided into three classes: (1) ox and heifer, (2) cow, (3) bull. Oxen and heifer hides produce the best results, forming aHeavy leathers.tough, tight, solid leather. Cow hides are thin, the hide itselfbeing fibrous, but still compact, and by reason of its spread or area is used chiefly for dressing purposes in the bag and portmanteau manufacture and work of a similar description. Bull hides are fibrous; they are largely used for heel lifts, and for cheap belting, the thicker hides being used in the iron and steel industry.
A second classification now presents itself, viz. the British home supply, continental (Europe), British colonial, South American, East Indian, Chinese, &c.
In the British home supply there are three chief breeds: (1) Shorthorns (Scotch breed), (2) Herefords (Midland breed), (3) Lowland, or Dutch class. From a tanner’s standpoint, the shorthorns are the best hides procurable. The cattle are exposed to a variable climate in the mountainous districts of Scotland, and nature, adapting herself to circumstances, provides them with a thicker and more compact hide; they are well grown, have short necks and small heads. The Hereford class are probably the best English hide; they likewise have small heads and horns, and produce good solid sole leather. The Lowland hides come chiefly from Suffolk, Kent and Surrey; the animals have long legs, long necks and big heads. The hides are usually thin and spready. The hides of the animals killed for the Christmas season are poor. The animals being stall-fed for the beef, the hides become distended, thin and surcharged with fat, which renders them unsuitable for first-class work.
The continental supply may be divided into two classes: (1) Hides from hilly regions, (2) hides from lowlands. All animals subject to strong winds and a wide range of temperatures have a very strong hide, and for this reason those bred in hilly and mountainous districts are best. The hides coming under heading No. 1 are of this class, and include those from the Swiss and Italian Alps, Bavarian Highlands and Pyrenees, also Florence, Oporto and Lisbon hides. They are magnificent hides, thick, tightly-built, and of smooth grain. The butt is long and the legs short. A serious defect in some of these hides is a thick place on the neck caused by the yoke; this part of the hide is absolute waste. Another defect, specially noticeable in Lisbon and Oporto hides, is goad marks on the rump, barbed wire scratches and warbles, caused by the gadfly. Those hides coming under heading No. 2 are Dutch, Rhine valley, Danish, Swedish, Norwegian, Hungarian, &c. The first three hides are very similar; they are spready, poorly grown, and are best used for bag and portmanteau work. Hungarian oxen are immense animals, and supply a very heavy bend. Swedish and Norwegian hides are evenly grown and of good texture; they are well flayed, and used a great deal for manufacturing picker bands, which require an even leather.
New Zealand, Australian and Queensland hides resemble good English. A small quantity of Canadian steers are imported; these are generally branded.
Chinese hides are exported dry, and they have generally suffered more or less from peptonization in the storing and drying; this cannot be detected until they are in the pits, when they fall to pieces.
Anglos are imported as live-stock, and are killed within forty-eight hours. They come to Hull, Birkenhead, Avonmouth and Deptford from various American ports, and usually give a flatter result than English, the general quality depending largely on whether the ship has had a good voyage or not.
Among South American hides, Liebig’s slaughter supply the best; they are thoroughly clean and carefully trimmed and flayed. They come to London, Antwerp and Havre, and except for being branded are of first-class quality. Second to the Liebig slaughter come the Uruguay hides.
East Indian hides are known as kips, and are supposed to be, and should be, the hides of yearling cattle. They are now dressed to a large extent in imitation of box calf, being much cheaper. They come from a small breed of ox, and have an extremely tight grain; the leather is not so soft as calf.
Calf-skins are largely supplied by the continent. They are soft and pliant, and have a characteristically fine grain, are tight in texture and quite apart from any other kind of skin.
The most valuable part of a sheepskin is the wool, and the value of the pelt is inversely as the value of the wool. Pure Leicester and Norfolk wools are very valuable, and next is the North and South Downs, but the skins,i.e.theLight leathers.pelts, of these animals are extremely poor. Devon and Cheviot cross-bred sheep supply a fair pelt, and sometimes these sheep are so many times crossed that it is quite impossible to tell what the skin is. Welsh skins also supply a good tough pelt, though small. Indian and Persian sheepskins are very goaty, the herds being allowed to roam about together so much. The sheepskin is the most porous and open-textured skin in existence, as also the most greasy one; it is flabby and soft, with a tight, compact grain, but an extremely loose flesh. Stillborn lambs and lambs not over a month old are worth much more than when they have lived for three months; they are used for the manufacture of best kid gloves, and must be milk skins. Once the lambs have taken to grass the skins supply a harsher leather.
The best goat-skins come from the Saxon and Bavarian Highlands, Swiss Alps, Pyrenees, Turkey, Bosnia, Southern Hungary and the Urals. The goats being exposed to all winds yield fine skins. A good number come from Argentina and from Abyssinia, the Cape and other parts of Africa. Of all light leathers the goat has the toughest and tightest grain; it is, therefore, especially liked for fancy work. The grain is rather too bold for glacé work, for which the sheep is largely used.
The seal-skin, used largely for levant work, is the skin of the yellow-hair seal, found in the Northern seas, the Baltic, Norway and Sweden, &c. The skin has a large, bold, brilliant grain, and being a large skin is much used for upholstery and coach work, like the Cape goat. It is quite distinct from the fur seal.
Porpoise hide is really the hide of the white whale; it is dressed for shooting, fishing and hunting boots. Horse hide is dressed for light split and upper work; being so much stall-fed it supplies only a thin, spready leather. The skins of other Equidae, such as the ass, zebra, quagga, &c. are also dressed to some small extent, but are not important sources.
Structure of Skin.—Upon superficial inspection, the hides and skins of all mammalia appear to be unlike each other in general structure, yet, upon closer examination, it is found that the anatomical structure of most skins is so similar that for all practical purposes we may assume that there is no distinction (seeSkin and Exo-skeleton). But from the practical point of view, as opposed to the anatomical, there are great and very important differences, such as those of texture, thickness, area, &c.; and these differences cause a great divergence in the methods of tanning used, almost necessitating a distinct tannage for nearly every class of hide or skin.The skins of the lower animals, such as alligators, lizards, fish and snakes, differ to a large extent from those of the mammalia, chiefly in the epidermis, which is much more horny in structure and forms scales.The skin is divided into two distinct layers: (1) the epidermis or epithelium,i.e.the cuticle, (2) the corium derma, or cutis,i.e.the true skin. These two layers are not only different in structure, but are also of entirely distinct origin. The epidermis again divides itself into two parts, viz. the “horny layer” or surface skin, and therete Malpighi, named after the Italian anatomist who first drew attention to its existence. Therete Malpighiis composed of living, soft, nucleated cells, which multiply by division, and, as they increase, are gradually pushed to the surface of the skin, becoming flatter and drier as they near it, until they reach the surface as dried scales. The epidermis is thus of cellular structure, and more or less horny or waterproof. It must consequently be removed together with the hair, wool or bristles before tannage begins, but as it is very thin compared with the corium, this matters little.The hair itself does not enter the corium, but is embedded in a sheath of epidermic structure, which is part of and continuous with the epidermis. It is of cellular structure, and the fibrous part is composed of long needle-shaped cells which contain the pigment with which the hair is coloured. Upon removal of the hair some of these cells remain behind and colour the skin, and this colour does not disappear until these cells are removed by scudding. Each hair is supplied with at least two fat or sebaceous glands, which discharge into the orifice of the hair sheath; these glands impart to the hair that natural glossy appearance which is characteristic of good health. The hair bulb (b, fig. 1) consists of living nucleated cells, which multiply rapidly, and, like therete Malpighi, cause an upward pressure, getting harder at the same time, thereby lengthening the hair.The hair papilla (a, fig. 1) consists of a globule of the corium or true skin embedded in the hair bulb, which by means of blood-vessels feeds and nourishes the hair. Connected with the lower part of each hair is an oblique muscle known as the arrector or erector pili, seen atk, fig. 1; this is an involuntary muscle, and is contracted by sudden cold, heat or shock, with an accompanying tightening of the skin, producing the phenomenon commonly known as “goose flesh.” This is the outcome of the contracted muscle pulling on the base of the hair, thereby giving it a tendency to approach the vertical, and producing the simultaneous effect of making the “hair stand on end.”The sudoriferous or sweat glands (R, fig. 1) consist of long spiral-like capillaries, formed from the fibres of the connective tissue of the corium. These glands discharge sometimes directly through the epidermis, but more often into the orifice of the hair-sheath.The epidermis is separated from the corium by a very important and very fine membrane, termed the “hyaline” or “glassy layer,” which constitutes the actual grain surface of a hide or skin. This layer is chemically different from the corium, as if it is torn or scratched during the process of tanning the colour of the underlying parts is much lighter than that of the grain surface.Fig. 1.a, Hair papilla.b, Hair bulb.c, Hair sheath showing epidermic structure.d, Dermic coat of hair sheath.e, Outer root sheath.f, Inner root sheath.g, Hair cuticle.h, Hair.J, Sebaceous glands.k, Erector pili.m, Sweat ducts.nandp, Epidermis.n, Rete Malpighi.p, Horny layer.R, Sweat or sudoriferous gland.S, Opening at sweat duct.The corium, unlike the epidermis, is of fibrous, not cellular structure; moreover, the fibres do not multiply among themselves, but are gradually developed as needed from the interfibrillar substance, a semi-soluble gelatinous modification of the true fibre. This interfibrillar substance consequently has no structure, and is prepared at any time on coming into contact with tannin to form amorphous leather, which fills what would in the absence of this substance be interfibrillar spaces. The more of this matter there is present the more completely will the spaces be filled, and the more waterproof will be the leather. An old bull, as is well known, supplies a very poor, soft and spongy leather, simply because the hide lacks interfibrillar substance, which has been sapped up by the body. The fibres are, therefore, separated by interfibrillar spaces, which on contact with water absorb it with avidity by capillary attraction. But a heifer hide or young calf supplies the most tight and waterproof leather known, because the animals are young, and having plenty of nourishment do not require to draw upon and sap the interfibrillar substance with which the skin is full to overflowing.The corium obtains its food from the body by means of lymph ducts, with which it is well supplied. It is also provided with nodules of lymph to nourish the hair, and nodules of grease, which increase in number as they near the flesh side, until the net skin,panniculus adiposus, or that which separates the corium from meat proper, is quite full with them.The corium is coarse in the centre of the skin where the fibres, which are of the kind known as white connective tissue, and which exist in bundles bound together with yellow elastic fibres, are loosely woven, but towards the flesh side they become more compact, and as the hyaline layer is neared the bundles of fibres get finer and finer, and are much more tightly interwoven, until finally, next the grain itself, the fibres no longer exist in bundles, but as individual fibrils lying parallel with the grain. This layer is known as thepars papillaris. The bundles of fibre interweave one another in every conceivable direction. The fibrils are extremely minute, and are cemented together with a medium rather more soluble than themselves.There are only two exceptions to this general structure which need be taken into account. Sheep-skin is especially loosely woven in the centre, so much so that any carelessness in the wet work or sweating process enables one to split the skin in two by tearing. This loosely-woven part is full of fatty nodules, and the skin is generally split at this part, the flesh going for chamois leather and the grain for skivers. The other notable exception is the horse hide, which has a third skin over the loins just above the kidneys, known as the crup; it is very greasy and tight in structure, and is used for making a very waterproof leather for seamen’s and fishermen’s boots. Pig-skin, perhaps, is rather peculiar, in the fact that the bristles penetrate almost right through the skin.Tanning Materials.—Tannin or tannic acid is abundantly formed in a very large number of plants, and secreted in such diverse organs and members as the bark, wood, roots, leaves, seed-pods, fruit, &c. The number of tannins which exists has not been determined, nor has the constitution of those which do exist been satisfactorily settled. As used in the tanyard tannin is present both in the free state and combined with colouring matter and accompanied by decomposition products, such as gallic acid or phlobaphenes (anhydrides of the tannins), respectively depending upon the series to which the tannin belongs. In whatever other points they differ, they all have the common property of being powerfully astringent, of forming insoluble compounds with gelatine or gelatinous tissue, of being soluble in water to a greater or lesser extent, and of forming blacks (greenish or bluish) with iron. Pyrogallol tannins give a blue-black coloration or precipitate with ferric salts, and catechol tannins a green-black; and whereas bromine water gives a precipitate with catechol tannins, it does not with pyrogallol tannins. There are two distinctive classes of tannins, viz. catechol and pyrogallol tannins. The materials belonging to the former series are generally much darker in colour than those classified with the latter, and moreover they yield reds, phlobaphenes or tannin anhydrides, which deposit on or in the leather. Pyrogallol tannins include some of the lightest coloured and best materials known, and, speaking generally, the leather produced by them is not so harsh or hard as that produced with catechol tannins. They decompose, yielding ellagic acid (known technically as “bloom”) and gallic acid; the former has waterproofing qualities, because it fills the leather, at the same time giving weight.It has been stated, and perhaps with some truth, that leather cannot be successfully made with catechol tannins alone; pyrogallol tannins, however, yield an excellent leather; but the finest results are obtained by blending the two.The classification of the chief tanning materials is as follows:—Pyrogallols.Catechols.Myrobalans (Terminalia Chebula).Gambier (Uncaria Gambir).Chestnut wood (Castanea vesca).Hemlock (Abies canadensis).Divi-divi (Caesalpinia Coriaria).Quebracho (Quebracho Colorado).Algarobilla (Caesalpinia brevifolia).Mangrove or Cutch (Rhizophora Mangle).Sumach (Rhus Coriaria).Mimosa or Golden Wattle (Acacia Pycnantha).Oakwood (Quercus family).Larch (Larix Europaea).Chestnut oak (Quercus Prinus).Canaigre (Rumer Hymenosepalum).Galls (Quercus Infectoria).Birch (Betula alba).Willow (Salix arenaria).Cutch Catechu (Acacia Catechu).Subsidiary.Oakbark (Quercus Robur).Valonia (Quercus Aegilops).Myrobalans are the fruit of an Indian tree. There are several different qualities, the order of which is as follows, the best being placed first: Bhimley, Jubbalpore, Rajpore, Fair Coast Madras and Vingorlas. They are a very light-coloured material, containing from 27 % to 38 % of tannin; they deposit much “bloom,” ferment fairly rapidly, supplying acidity, and yield a mellow leather.Chestnut comes on the market in the form of crude and decolorized liquid extracts, containing about 27 % to 31 % of tannin, and yields a good leather of a light-brown colour.Oakwood reaches the market in the same form; it is a very similar material, but only contains 24 % to 27 % of tannin, and yields a slightly heavier and darker leather.Divi-divi is the dried seed pods of an Indian tree containing 40 % to 45 % of tannin, and yielding a white leather; it might be valuable but for the tendency to dangerous fermentation and development of a dark-red colouring matter.Algarobilla consists of the seeds of an Indian tree, containing about 45 % of tannin, and in general properties is similar to divi-divi, but does not discolour so much upon fermentation.Sumach is perhaps the best and most useful material known. It is the ground leaves of a Sicilian plant, containing about 28 % of tannin, and yielding a nearly white and very beautiful leather. It is used alone for tanning the best moroccos and finer leather, and being so valuable is much adulterated, the chief adulterant beingPistacia lentiscus(Stinko or Lentisco), an inferior and light-coloured catechol tannin. Other but inferior sumachs are also used. There is Venetian sumach (Rhus cotinus) and Spanish sumach (Colpoon compressa); these are used to some extent in the countries bordering on the Mediterranean.R. GlabraandR. Copallinaare also used in considerable quantities in America, where they are cultivated.Galls are abnormal growths found upon oaks, and caused by the gall wasp laying eggs in the plant. They are best harvested just before the insect escapes. They contain from 50 % to 60 % of tannin, and are generally used for the commercial supply of tannic acid, and not for tanning purposes.Gambier, terra japonica or catechu, is the product of a shrub cultivated in Singapore and the Malay Archipelago. It is made by boiling the shrub and allowing the extract to solidify. It is apeculiar material, and may be completely washed out of a leather tanned with it. It mellows exceedingly, and keeps the leather fibre open; it may be said that it only goes in the leather to prepare and make easy the way for other tannins. Block gambier contains from 35 % to 40 % and cube gambier from 50 % to 65 % of tannin.Hemlock generally reaches the market as extract, prepared from the bark of the American tree. It contains about 22 % of tannin, has a pine-like odour, but yields a rather dark-coloured red leather.Quebracho is imported mainly as solid extract, containing 63 % to 70 % of tannin; it is a harsh, light-red tannage, but darkens rapidly on exposure to light. It is used for freshening up very mellow liquors, but is rather wasteful, as it deposits an enormous amount of its tannin as phlobaphenes.Mangrove or cutch is a solid extract prepared from the mangrove tree found in the swamps of Borneo and the Straits Settlements; it contains upwards of 60 % of a red tannin.Mimosa is the bark of the Australian golden wattle (Acacia pycnantha), and contains from 36 % to 50 % of tannin. It is a rather harsh tannage, yielding a flesh-coloured leather, and is useful for sharpening liquors. This bark is now successfully cultivated in Natal. The tannin content of this Natal bark is somewhat inferior, but the colour is superior to the Australian product.Larch bark contains 9 % to 10 % of light-coloured tannin, and is used especially for tanning Scotch basils.Canaigre is the air-dried tuberous roots of a Mexican plant, containing 25 % to 30 % of tannin and about 8 % of starch. It yields an orange-coloured leather of considerable weight and firmness. Its cultivation did not pay well enough, so that it is little used.Cutch, catechu or “dark catechu,” is obtained from the wood of Indian acacias, and is not to be confounded with mangrove cutch. It contains 60 % of tanning matter and a large proportion of catechin similar to that contained in gambier, but much redder. It is used for dyeing browns and blacks with chrome and iron mordants.The willow and the white birch barks contain, respectively, 12 % to 14 % and 2 % to 5 % of tannin. In combination they are used to produce the famous Russia leather, whose insect-resisting odour is due to the birch bark. In America this leather is imitated with the American black birch bark (Betula lenta), and also with the oil obtained from its dry distillation.In the list of materials two have been placed in a subsidiary class because they are a mixture of catechol and pyrogallol tannin. Oak bark produces the best leather known, proving that a blend of the two classes of tannins gives the best results. It is the bark of the coppice oak, and contains 12 % to 14 % of a reddish-yellow tannage. Valonia is the acorn cup of the Turkish and Greek oak. The Smyrna or Turkish valonia is best, and contains 32 % to 36 % of an almost white tannin. Greek valonia is greyer in colour, and contains 26 % to 30 % of tannin. It yields a tough, firm leather of great weight, due to the rapid deposition of a large amount of bloom.Grinding and Leaching1Tanning Materials.—At first sight it would not seem possible that science could direct such a clumsy process as the grinding of tanning materials, and yet even here, the “scientific smashing” of tanning materials may mean the difference between profit and loss to the tanner. In most materials the tannin exists imprisoned in cells, and is also to some extent free, but with this latter condition the science of grinding has nothing to do. If tanning materials are simply broken by a series of clean cuts, only those cells directly on the surfaces of the cuts will be ready to yield their tannin; therefore, if materials are ground by cutting, a proportion of the total tannin is thrown away. Hence it is necessary to bruise, break and otherwise sever the walls of all the cells containing the tannin; so that the machine wanted is one which crushes, twists and cuts the material at the same time, turning it out of uniform size and with little dust.The apparatus in most common use is built on the same principle as the coffee mill, which consists of a series of segmental cutters; as the bark works down into the smaller cutters of the mill it is twisted and cut in every direction. This is a very good form of mill, but it requires a considerable amount of power and works slowly. The teeth require constant renewal, and should, therefore, be replaceable in rows, not, as in some forms, cast on the bell. The disintegrator is another form of mill, which produces its effect by violent concussion, obtained by the revolution in opposite directions of from four to six large metal arms fitted with projecting spikes inside a drum, the faces of which are also fitted with protruding pieces of metal. The arms make from 2000 to 4000 revolutions per minute. The chief objection to this apparatus is that it forms much dust, which is caught in silken bags fitted to gratings in the drum. The myrobalans crusher, a very useful machine for such materials as myrobalans and valonia, consists of a pair of toothed rollers above and a pair of fluted rollers beneath. The material is dropped upon the toothed rollers first, where it is broken and crushed; then the crushing is finished and any sharp corners rounded off in the fluted rollers.It must not be thought that now the material is ground it is necessarily ready for leaching. This may or may not be so, depending upon whether the tanner is making light or heavy leathers. If light leathers are being considered, it is ready for immediate leaching,i.e.to be infused with water in preparation of a liquor. If heavy leathers are in process of manufacture, he would be a very wasteful tanner who would extract his material raw. It must be borne in mind that when an infusion is made with fresh tanning material, the liquor begins to deposit decomposition products after standing a day or two, and the object of the heavy-leather tanner is to get this material deposited in the leather, to fill the pores, produce weight and make a firm, tough product. With this end in view he dusts his hides with this fresh material in the layers,i.e.he spreads a layer between each hide as it is laid down, so that the strong liquors penetrate and deposit in the hides. When most of this power to deposit has been usefully utilized in the layers, then the material (which is now, perhaps, half spent) is leached. The light-leather maker does not want a hard, firm leather, but a soft and pliable product; hence he leaches his material fresh, and does not trouble as to whether the tannin deposits in the pits or not.Whether fresh or partially spent material is leached, the process is carried out in the same way. There are several methods in vogue; the best method only will be described, viz. the “press leach” system.The leaching is carried out in a series of six square pits, each holding about 3 to 4 tons of material. The method depends upon the fact that when a weak liquor is forced over a stronger one they do not mix, by reason of the higher specific gravity of the stronger one; the weaker liquor, therefore, by its weight forces the stronger liquor downwards, and as the pit in which it is contained is fitted with a false bottom and side duct running over into the next pit, the stronger liquor is forced upwards through this duct on to the next stronger pit. There the process is repeated, until finally the weak liquor or water, as the case may be, is run off the last vat as a very strong infusion. As a concrete example let us take the six pits shown in the figure.No. 6 is the last vat, and the liquor, which is very strong, is about to be run off. No. 1 is spent material, over which all six liquors have passed, the present liquor having been pumped on as fresh water. The liquor from No. 6 is run off into the pump well, and liquor No. 1 is pumped over No. 2, thus forcing all liquors one forward and leaving pit No. 1 empty; this pit is now cast and filled with clean fishings and perhaps a little new material, clean water is then pumped on No. 2, which is now the weakest pit, and all liquors are thus forced forward one pit more, making No. 1 the strongest pit. After infusing for some time this is run off to the pump well, and the process repeated. It may be noted that the hotter the water is pumped on the weakest pit, the better will the material be spent, and the nearer the water is to boiling-point the better; in fact, a well-managed tanyard should have the spent tan down to between 1% and 2% of tannin, although this material is frequently thrown away containing up to 10% and sometimes even more. There is a great saving of time and labour in this method, since the liquors are self-adjusting.Testing Tan Liquors.—The methods by which the tanning value of any substance may be determined are many, but few are at once capable of simple application and minute accuracy. An old method of ascertaining the strength of a tan liquor is by means of a hydrometer standardized against water, and called a barkometer. It consists of a long graduated stem fixed to a hollow bulb, the opposite end of which is weighted. It is placed in the liquor, the weighted end sinks to a certain depth, and the reading is taken on the stem at that point which touches “water mark.” The graduations are such that if the specific gravity is multiplied by 1000 and then 1000 is subtracted from the result, the barkometer strength of the liquor is obtained. Thus 1029 specific gravity equals 29° barkometer. This method affords no indication of the amount of tannin present, but is useful to the man who knows his liquors by frequent analysis.A factor which governs the quality of the leather quite as much as the tannin itself is the acidity of the liquors. It is known that gallic and tannic acids form insoluble calcium salts, and all the other acids present as acetic, propionic, butyric, lactic, formic, &c., form comparatively soluble salts, so that an easy method of determining this important factor is as follows:—Take a quantity, say 100 c.c., of tan liquor, filter till clear through paper, then pipette 10 c.c. into a small beaker (about 1½ in. diameter), place it on some printed paper and note how clear the print appears through the liquor; now gradually add from a burette a clear solution of saturated lime water until the liquor becomes just cloudy, that is until it just loses its brilliancy. Now read off the number of cubic centimetres required in the graduated stem of the burette, and either read as degrees (counting each c.c. as one degree), to which practice at once gives a useful signification, or calculate out in terms of acetic acid per 100 c.c. of liquor, reckoning saturated lime water as1⁄20normal.The methods which deal with the actual testing for tannin itselfdepend mostly upon one or other of two processes; either the precipitation of the tannin by means of gelatin, or its absorption by means of prepared hide. Sir Humphry Davy was the first to propose a method for analysing tanning materials, and he precipitated the tannin by means of gelatin in the presence of alum, then dried and weighed the precipitate, after washing free from excess of reagents. This method was improved by Stoddart, but cannot lay claim to much accuracy. Warington and Müller again modified the method, but their procedure being tedious and difficult to work could not be regarded as a great advance. Wagner then proposed precipitation by means of the alkaloids, with special regard to cinchonine sulphate in the presence of rosaniline acetate as indicator, but this method also proved useless. After this many metallic precipitants were tried, used gravimetrically and volumetrically, but without success. The weighing of precipitated tannates will never succeed, because the tannins are such a diverse class of substances that each tannin precipitates different quantities of the precipitants, and some materials contain two or three different tannins. Then there are also the difficulties of incomplete precipitation and the precipitation of colouring matter, &c. Among this class of methods may be mentioned Garland’s, in which tartar emetic and sal ammoniac were employed. It was improved by Richards and Palmer.Another class of methods depends upon the destruction of the tannin by some oxidizing agent, and the estimation of the amount required. Terreil rendered the tannin alkaline, and after agitating it with a known quantity of air, estimated the volume of oxygen absorbed. The method was slow and subject to many sources of error. Commaille oxidized with a known quantity of iodic acid and estimated the excess of iodate. This process also was troublesome, besides oxidizing the gallic acid (as do all the oxidation processes), and entailing a separate estimation of them after the removal of the tannin. Ferdinand Jean (1877) titrated alkaline tannin solution with standard iodine, but the mixture was so dark that the end reaction with starch could not be seen; in addition the gallic acid had again to be estimated. Monier proposed permanganate as an oxidizing agent, and Lowenthal made a very valuable improvement by adding indigo solution to the tannin solution, which controlled the oxidation and acted as indicator. This method also required double titration because of the gallic acid present, the tanning matters being removed from solution by means of gelatin and acidified salt.The indirect gravimetric hide-powder method first took form about 1886. It was published inDer Gerberby Simand and Weiss, other workers being Eitner and Meerkatz. Hammer, Muntz and Ramspacher did some earlier work on similar lines, depending upon the specific gravity of solutions. Professor H. R. Procter perfected this method by packing a bell, similar in shape to a bottomless bottle of about 2 oz. (liq.) capacity, with the hide-powder, and siphoning the tan liquor up through the powder and over into a receiver. This deprives the tan liquor of tannin, and a portion of this non-tannin solution is evaporated to dryness and weighed till constant; similarly a portion of the original solution containing non-tannins and tannins is evaporated and weighed till constant; then the weight of the non-tannins subtracted from the weight of the non-tannins and tannins gives the weight of tannin, which is calculated to percentage on original solutions. This method was adopted as official by the International Association of Leather Trades Chemists until September 1906, when its faults were vividly brought before them by Gordon Parker of London and Bennett of Leeds, working in collaboration, although other but not so complete work had been previously done to the same end. The main faults of the method were that the hide-powder absorbed non-tannins, and therefore registered them as tannins, and the hide-powder was partially soluble. This difficulty has now been overcome to a large extent in the present official method of the I.A.L.T.C.Meanwhile, Parker and Munro Payne proposed a new method of analysis, the essence of which is as follows:—A definite excess of lime solution is added to a definite quantity of tannin solution and the excess of lime estimated; the tan solution is now deprived of tannin by means of a soluble modification of gelatin, called “collin,” and the process is repeated. Thus we get two sets of figures, viz. total absorption and acid absorption (i.e.acids other than tan); the latter subtracted from the former gives tannin absorption, and this is calculated out in percentage of original liquor. The method failed theoretically, because a definite molecular weight had to be assumed for tannins which are all different. There are also several other objections, but though, like the hide-powder method, it is quite empirical, it gives exceedingly useful results if the rules for working are strictly adhered to.The present official method of the I.A.L.T.C. is a modification of the American official method, which is in turn a modification of a method proposed by W. Eitner, of the Vienna Leather Research Station. The hide-powder is very slightly chrome-tanned with a basic solution of chromium chloride, 2 grammes of the latter being used per 100 grammes of hide-powder, and is then washed free from soluble salts and squeezed to contain 70% of moisture, and is ready for use. This preliminary chroming does away with the difficulty of the powder being soluble, by rendering it quite insoluble; it also lessens the tendency to absorb non-tannins. Such a quantity of this wet powder as contains 6.5 grammes of dry hide is now taken, and water is added until this quantity contains exactly 20 grammes of moisture,i.e.26.5 grammes in all; it is then agitated for 15 minutes with 100 c.c. of the prepared tannin solution, which is made up to contain tannin within certain definite limits, in a mechanical rotator, and filtered. Of this non-tannin solution 50 c.c. is then evaporated to dryness. The same thing is done with 50 c.c. of original solution containing non-tannins and tannins, and both residues are weighed. The tannin is thus determined by difference. The method does all that science can do at present. The rules for carrying out the analysis are necessarily very strict. The object in view is that all chemists should get exactly concordant results, and in this the I.A.L.T.C. has succeeded.The work done by Wood, Trotman, Procter, Parker and others on the alkaloidal precipitation of tannin deserves mention.
Structure of Skin.—Upon superficial inspection, the hides and skins of all mammalia appear to be unlike each other in general structure, yet, upon closer examination, it is found that the anatomical structure of most skins is so similar that for all practical purposes we may assume that there is no distinction (seeSkin and Exo-skeleton). But from the practical point of view, as opposed to the anatomical, there are great and very important differences, such as those of texture, thickness, area, &c.; and these differences cause a great divergence in the methods of tanning used, almost necessitating a distinct tannage for nearly every class of hide or skin.
The skins of the lower animals, such as alligators, lizards, fish and snakes, differ to a large extent from those of the mammalia, chiefly in the epidermis, which is much more horny in structure and forms scales.
The skin is divided into two distinct layers: (1) the epidermis or epithelium,i.e.the cuticle, (2) the corium derma, or cutis,i.e.the true skin. These two layers are not only different in structure, but are also of entirely distinct origin. The epidermis again divides itself into two parts, viz. the “horny layer” or surface skin, and therete Malpighi, named after the Italian anatomist who first drew attention to its existence. Therete Malpighiis composed of living, soft, nucleated cells, which multiply by division, and, as they increase, are gradually pushed to the surface of the skin, becoming flatter and drier as they near it, until they reach the surface as dried scales. The epidermis is thus of cellular structure, and more or less horny or waterproof. It must consequently be removed together with the hair, wool or bristles before tannage begins, but as it is very thin compared with the corium, this matters little.
The hair itself does not enter the corium, but is embedded in a sheath of epidermic structure, which is part of and continuous with the epidermis. It is of cellular structure, and the fibrous part is composed of long needle-shaped cells which contain the pigment with which the hair is coloured. Upon removal of the hair some of these cells remain behind and colour the skin, and this colour does not disappear until these cells are removed by scudding. Each hair is supplied with at least two fat or sebaceous glands, which discharge into the orifice of the hair sheath; these glands impart to the hair that natural glossy appearance which is characteristic of good health. The hair bulb (b, fig. 1) consists of living nucleated cells, which multiply rapidly, and, like therete Malpighi, cause an upward pressure, getting harder at the same time, thereby lengthening the hair.
The hair papilla (a, fig. 1) consists of a globule of the corium or true skin embedded in the hair bulb, which by means of blood-vessels feeds and nourishes the hair. Connected with the lower part of each hair is an oblique muscle known as the arrector or erector pili, seen atk, fig. 1; this is an involuntary muscle, and is contracted by sudden cold, heat or shock, with an accompanying tightening of the skin, producing the phenomenon commonly known as “goose flesh.” This is the outcome of the contracted muscle pulling on the base of the hair, thereby giving it a tendency to approach the vertical, and producing the simultaneous effect of making the “hair stand on end.”
The sudoriferous or sweat glands (R, fig. 1) consist of long spiral-like capillaries, formed from the fibres of the connective tissue of the corium. These glands discharge sometimes directly through the epidermis, but more often into the orifice of the hair-sheath.
The epidermis is separated from the corium by a very important and very fine membrane, termed the “hyaline” or “glassy layer,” which constitutes the actual grain surface of a hide or skin. This layer is chemically different from the corium, as if it is torn or scratched during the process of tanning the colour of the underlying parts is much lighter than that of the grain surface.
a, Hair papilla.
b, Hair bulb.
c, Hair sheath showing epidermic structure.
d, Dermic coat of hair sheath.
e, Outer root sheath.
f, Inner root sheath.
g, Hair cuticle.
h, Hair.
J, Sebaceous glands.
k, Erector pili.
m, Sweat ducts.
nandp, Epidermis.
n, Rete Malpighi.
p, Horny layer.
R, Sweat or sudoriferous gland.
S, Opening at sweat duct.
The corium, unlike the epidermis, is of fibrous, not cellular structure; moreover, the fibres do not multiply among themselves, but are gradually developed as needed from the interfibrillar substance, a semi-soluble gelatinous modification of the true fibre. This interfibrillar substance consequently has no structure, and is prepared at any time on coming into contact with tannin to form amorphous leather, which fills what would in the absence of this substance be interfibrillar spaces. The more of this matter there is present the more completely will the spaces be filled, and the more waterproof will be the leather. An old bull, as is well known, supplies a very poor, soft and spongy leather, simply because the hide lacks interfibrillar substance, which has been sapped up by the body. The fibres are, therefore, separated by interfibrillar spaces, which on contact with water absorb it with avidity by capillary attraction. But a heifer hide or young calf supplies the most tight and waterproof leather known, because the animals are young, and having plenty of nourishment do not require to draw upon and sap the interfibrillar substance with which the skin is full to overflowing.
The corium obtains its food from the body by means of lymph ducts, with which it is well supplied. It is also provided with nodules of lymph to nourish the hair, and nodules of grease, which increase in number as they near the flesh side, until the net skin,panniculus adiposus, or that which separates the corium from meat proper, is quite full with them.
The corium is coarse in the centre of the skin where the fibres, which are of the kind known as white connective tissue, and which exist in bundles bound together with yellow elastic fibres, are loosely woven, but towards the flesh side they become more compact, and as the hyaline layer is neared the bundles of fibres get finer and finer, and are much more tightly interwoven, until finally, next the grain itself, the fibres no longer exist in bundles, but as individual fibrils lying parallel with the grain. This layer is known as thepars papillaris. The bundles of fibre interweave one another in every conceivable direction. The fibrils are extremely minute, and are cemented together with a medium rather more soluble than themselves.
There are only two exceptions to this general structure which need be taken into account. Sheep-skin is especially loosely woven in the centre, so much so that any carelessness in the wet work or sweating process enables one to split the skin in two by tearing. This loosely-woven part is full of fatty nodules, and the skin is generally split at this part, the flesh going for chamois leather and the grain for skivers. The other notable exception is the horse hide, which has a third skin over the loins just above the kidneys, known as the crup; it is very greasy and tight in structure, and is used for making a very waterproof leather for seamen’s and fishermen’s boots. Pig-skin, perhaps, is rather peculiar, in the fact that the bristles penetrate almost right through the skin.
Tanning Materials.—Tannin or tannic acid is abundantly formed in a very large number of plants, and secreted in such diverse organs and members as the bark, wood, roots, leaves, seed-pods, fruit, &c. The number of tannins which exists has not been determined, nor has the constitution of those which do exist been satisfactorily settled. As used in the tanyard tannin is present both in the free state and combined with colouring matter and accompanied by decomposition products, such as gallic acid or phlobaphenes (anhydrides of the tannins), respectively depending upon the series to which the tannin belongs. In whatever other points they differ, they all have the common property of being powerfully astringent, of forming insoluble compounds with gelatine or gelatinous tissue, of being soluble in water to a greater or lesser extent, and of forming blacks (greenish or bluish) with iron. Pyrogallol tannins give a blue-black coloration or precipitate with ferric salts, and catechol tannins a green-black; and whereas bromine water gives a precipitate with catechol tannins, it does not with pyrogallol tannins. There are two distinctive classes of tannins, viz. catechol and pyrogallol tannins. The materials belonging to the former series are generally much darker in colour than those classified with the latter, and moreover they yield reds, phlobaphenes or tannin anhydrides, which deposit on or in the leather. Pyrogallol tannins include some of the lightest coloured and best materials known, and, speaking generally, the leather produced by them is not so harsh or hard as that produced with catechol tannins. They decompose, yielding ellagic acid (known technically as “bloom”) and gallic acid; the former has waterproofing qualities, because it fills the leather, at the same time giving weight.
It has been stated, and perhaps with some truth, that leather cannot be successfully made with catechol tannins alone; pyrogallol tannins, however, yield an excellent leather; but the finest results are obtained by blending the two.
The classification of the chief tanning materials is as follows:—
Subsidiary.
Oakbark (Quercus Robur).Valonia (Quercus Aegilops).
Oakbark (Quercus Robur).
Valonia (Quercus Aegilops).
Myrobalans are the fruit of an Indian tree. There are several different qualities, the order of which is as follows, the best being placed first: Bhimley, Jubbalpore, Rajpore, Fair Coast Madras and Vingorlas. They are a very light-coloured material, containing from 27 % to 38 % of tannin; they deposit much “bloom,” ferment fairly rapidly, supplying acidity, and yield a mellow leather.
Chestnut comes on the market in the form of crude and decolorized liquid extracts, containing about 27 % to 31 % of tannin, and yields a good leather of a light-brown colour.
Oakwood reaches the market in the same form; it is a very similar material, but only contains 24 % to 27 % of tannin, and yields a slightly heavier and darker leather.
Divi-divi is the dried seed pods of an Indian tree containing 40 % to 45 % of tannin, and yielding a white leather; it might be valuable but for the tendency to dangerous fermentation and development of a dark-red colouring matter.
Algarobilla consists of the seeds of an Indian tree, containing about 45 % of tannin, and in general properties is similar to divi-divi, but does not discolour so much upon fermentation.
Sumach is perhaps the best and most useful material known. It is the ground leaves of a Sicilian plant, containing about 28 % of tannin, and yielding a nearly white and very beautiful leather. It is used alone for tanning the best moroccos and finer leather, and being so valuable is much adulterated, the chief adulterant beingPistacia lentiscus(Stinko or Lentisco), an inferior and light-coloured catechol tannin. Other but inferior sumachs are also used. There is Venetian sumach (Rhus cotinus) and Spanish sumach (Colpoon compressa); these are used to some extent in the countries bordering on the Mediterranean.R. GlabraandR. Copallinaare also used in considerable quantities in America, where they are cultivated.
Galls are abnormal growths found upon oaks, and caused by the gall wasp laying eggs in the plant. They are best harvested just before the insect escapes. They contain from 50 % to 60 % of tannin, and are generally used for the commercial supply of tannic acid, and not for tanning purposes.
Gambier, terra japonica or catechu, is the product of a shrub cultivated in Singapore and the Malay Archipelago. It is made by boiling the shrub and allowing the extract to solidify. It is apeculiar material, and may be completely washed out of a leather tanned with it. It mellows exceedingly, and keeps the leather fibre open; it may be said that it only goes in the leather to prepare and make easy the way for other tannins. Block gambier contains from 35 % to 40 % and cube gambier from 50 % to 65 % of tannin.
Hemlock generally reaches the market as extract, prepared from the bark of the American tree. It contains about 22 % of tannin, has a pine-like odour, but yields a rather dark-coloured red leather.
Quebracho is imported mainly as solid extract, containing 63 % to 70 % of tannin; it is a harsh, light-red tannage, but darkens rapidly on exposure to light. It is used for freshening up very mellow liquors, but is rather wasteful, as it deposits an enormous amount of its tannin as phlobaphenes.
Mangrove or cutch is a solid extract prepared from the mangrove tree found in the swamps of Borneo and the Straits Settlements; it contains upwards of 60 % of a red tannin.
Mimosa is the bark of the Australian golden wattle (Acacia pycnantha), and contains from 36 % to 50 % of tannin. It is a rather harsh tannage, yielding a flesh-coloured leather, and is useful for sharpening liquors. This bark is now successfully cultivated in Natal. The tannin content of this Natal bark is somewhat inferior, but the colour is superior to the Australian product.
Larch bark contains 9 % to 10 % of light-coloured tannin, and is used especially for tanning Scotch basils.
Canaigre is the air-dried tuberous roots of a Mexican plant, containing 25 % to 30 % of tannin and about 8 % of starch. It yields an orange-coloured leather of considerable weight and firmness. Its cultivation did not pay well enough, so that it is little used.
Cutch, catechu or “dark catechu,” is obtained from the wood of Indian acacias, and is not to be confounded with mangrove cutch. It contains 60 % of tanning matter and a large proportion of catechin similar to that contained in gambier, but much redder. It is used for dyeing browns and blacks with chrome and iron mordants.
The willow and the white birch barks contain, respectively, 12 % to 14 % and 2 % to 5 % of tannin. In combination they are used to produce the famous Russia leather, whose insect-resisting odour is due to the birch bark. In America this leather is imitated with the American black birch bark (Betula lenta), and also with the oil obtained from its dry distillation.
In the list of materials two have been placed in a subsidiary class because they are a mixture of catechol and pyrogallol tannin. Oak bark produces the best leather known, proving that a blend of the two classes of tannins gives the best results. It is the bark of the coppice oak, and contains 12 % to 14 % of a reddish-yellow tannage. Valonia is the acorn cup of the Turkish and Greek oak. The Smyrna or Turkish valonia is best, and contains 32 % to 36 % of an almost white tannin. Greek valonia is greyer in colour, and contains 26 % to 30 % of tannin. It yields a tough, firm leather of great weight, due to the rapid deposition of a large amount of bloom.
Grinding and Leaching1Tanning Materials.—At first sight it would not seem possible that science could direct such a clumsy process as the grinding of tanning materials, and yet even here, the “scientific smashing” of tanning materials may mean the difference between profit and loss to the tanner. In most materials the tannin exists imprisoned in cells, and is also to some extent free, but with this latter condition the science of grinding has nothing to do. If tanning materials are simply broken by a series of clean cuts, only those cells directly on the surfaces of the cuts will be ready to yield their tannin; therefore, if materials are ground by cutting, a proportion of the total tannin is thrown away. Hence it is necessary to bruise, break and otherwise sever the walls of all the cells containing the tannin; so that the machine wanted is one which crushes, twists and cuts the material at the same time, turning it out of uniform size and with little dust.
The apparatus in most common use is built on the same principle as the coffee mill, which consists of a series of segmental cutters; as the bark works down into the smaller cutters of the mill it is twisted and cut in every direction. This is a very good form of mill, but it requires a considerable amount of power and works slowly. The teeth require constant renewal, and should, therefore, be replaceable in rows, not, as in some forms, cast on the bell. The disintegrator is another form of mill, which produces its effect by violent concussion, obtained by the revolution in opposite directions of from four to six large metal arms fitted with projecting spikes inside a drum, the faces of which are also fitted with protruding pieces of metal. The arms make from 2000 to 4000 revolutions per minute. The chief objection to this apparatus is that it forms much dust, which is caught in silken bags fitted to gratings in the drum. The myrobalans crusher, a very useful machine for such materials as myrobalans and valonia, consists of a pair of toothed rollers above and a pair of fluted rollers beneath. The material is dropped upon the toothed rollers first, where it is broken and crushed; then the crushing is finished and any sharp corners rounded off in the fluted rollers.
It must not be thought that now the material is ground it is necessarily ready for leaching. This may or may not be so, depending upon whether the tanner is making light or heavy leathers. If light leathers are being considered, it is ready for immediate leaching,i.e.to be infused with water in preparation of a liquor. If heavy leathers are in process of manufacture, he would be a very wasteful tanner who would extract his material raw. It must be borne in mind that when an infusion is made with fresh tanning material, the liquor begins to deposit decomposition products after standing a day or two, and the object of the heavy-leather tanner is to get this material deposited in the leather, to fill the pores, produce weight and make a firm, tough product. With this end in view he dusts his hides with this fresh material in the layers,i.e.he spreads a layer between each hide as it is laid down, so that the strong liquors penetrate and deposit in the hides. When most of this power to deposit has been usefully utilized in the layers, then the material (which is now, perhaps, half spent) is leached. The light-leather maker does not want a hard, firm leather, but a soft and pliable product; hence he leaches his material fresh, and does not trouble as to whether the tannin deposits in the pits or not.
Whether fresh or partially spent material is leached, the process is carried out in the same way. There are several methods in vogue; the best method only will be described, viz. the “press leach” system.
The leaching is carried out in a series of six square pits, each holding about 3 to 4 tons of material. The method depends upon the fact that when a weak liquor is forced over a stronger one they do not mix, by reason of the higher specific gravity of the stronger one; the weaker liquor, therefore, by its weight forces the stronger liquor downwards, and as the pit in which it is contained is fitted with a false bottom and side duct running over into the next pit, the stronger liquor is forced upwards through this duct on to the next stronger pit. There the process is repeated, until finally the weak liquor or water, as the case may be, is run off the last vat as a very strong infusion. As a concrete example let us take the six pits shown in the figure.
No. 6 is the last vat, and the liquor, which is very strong, is about to be run off. No. 1 is spent material, over which all six liquors have passed, the present liquor having been pumped on as fresh water. The liquor from No. 6 is run off into the pump well, and liquor No. 1 is pumped over No. 2, thus forcing all liquors one forward and leaving pit No. 1 empty; this pit is now cast and filled with clean fishings and perhaps a little new material, clean water is then pumped on No. 2, which is now the weakest pit, and all liquors are thus forced forward one pit more, making No. 1 the strongest pit. After infusing for some time this is run off to the pump well, and the process repeated. It may be noted that the hotter the water is pumped on the weakest pit, the better will the material be spent, and the nearer the water is to boiling-point the better; in fact, a well-managed tanyard should have the spent tan down to between 1% and 2% of tannin, although this material is frequently thrown away containing up to 10% and sometimes even more. There is a great saving of time and labour in this method, since the liquors are self-adjusting.
Testing Tan Liquors.—The methods by which the tanning value of any substance may be determined are many, but few are at once capable of simple application and minute accuracy. An old method of ascertaining the strength of a tan liquor is by means of a hydrometer standardized against water, and called a barkometer. It consists of a long graduated stem fixed to a hollow bulb, the opposite end of which is weighted. It is placed in the liquor, the weighted end sinks to a certain depth, and the reading is taken on the stem at that point which touches “water mark.” The graduations are such that if the specific gravity is multiplied by 1000 and then 1000 is subtracted from the result, the barkometer strength of the liquor is obtained. Thus 1029 specific gravity equals 29° barkometer. This method affords no indication of the amount of tannin present, but is useful to the man who knows his liquors by frequent analysis.
A factor which governs the quality of the leather quite as much as the tannin itself is the acidity of the liquors. It is known that gallic and tannic acids form insoluble calcium salts, and all the other acids present as acetic, propionic, butyric, lactic, formic, &c., form comparatively soluble salts, so that an easy method of determining this important factor is as follows:—
Take a quantity, say 100 c.c., of tan liquor, filter till clear through paper, then pipette 10 c.c. into a small beaker (about 1½ in. diameter), place it on some printed paper and note how clear the print appears through the liquor; now gradually add from a burette a clear solution of saturated lime water until the liquor becomes just cloudy, that is until it just loses its brilliancy. Now read off the number of cubic centimetres required in the graduated stem of the burette, and either read as degrees (counting each c.c. as one degree), to which practice at once gives a useful signification, or calculate out in terms of acetic acid per 100 c.c. of liquor, reckoning saturated lime water as1⁄20normal.
The methods which deal with the actual testing for tannin itselfdepend mostly upon one or other of two processes; either the precipitation of the tannin by means of gelatin, or its absorption by means of prepared hide. Sir Humphry Davy was the first to propose a method for analysing tanning materials, and he precipitated the tannin by means of gelatin in the presence of alum, then dried and weighed the precipitate, after washing free from excess of reagents. This method was improved by Stoddart, but cannot lay claim to much accuracy. Warington and Müller again modified the method, but their procedure being tedious and difficult to work could not be regarded as a great advance. Wagner then proposed precipitation by means of the alkaloids, with special regard to cinchonine sulphate in the presence of rosaniline acetate as indicator, but this method also proved useless. After this many metallic precipitants were tried, used gravimetrically and volumetrically, but without success. The weighing of precipitated tannates will never succeed, because the tannins are such a diverse class of substances that each tannin precipitates different quantities of the precipitants, and some materials contain two or three different tannins. Then there are also the difficulties of incomplete precipitation and the precipitation of colouring matter, &c. Among this class of methods may be mentioned Garland’s, in which tartar emetic and sal ammoniac were employed. It was improved by Richards and Palmer.
Another class of methods depends upon the destruction of the tannin by some oxidizing agent, and the estimation of the amount required. Terreil rendered the tannin alkaline, and after agitating it with a known quantity of air, estimated the volume of oxygen absorbed. The method was slow and subject to many sources of error. Commaille oxidized with a known quantity of iodic acid and estimated the excess of iodate. This process also was troublesome, besides oxidizing the gallic acid (as do all the oxidation processes), and entailing a separate estimation of them after the removal of the tannin. Ferdinand Jean (1877) titrated alkaline tannin solution with standard iodine, but the mixture was so dark that the end reaction with starch could not be seen; in addition the gallic acid had again to be estimated. Monier proposed permanganate as an oxidizing agent, and Lowenthal made a very valuable improvement by adding indigo solution to the tannin solution, which controlled the oxidation and acted as indicator. This method also required double titration because of the gallic acid present, the tanning matters being removed from solution by means of gelatin and acidified salt.
The indirect gravimetric hide-powder method first took form about 1886. It was published inDer Gerberby Simand and Weiss, other workers being Eitner and Meerkatz. Hammer, Muntz and Ramspacher did some earlier work on similar lines, depending upon the specific gravity of solutions. Professor H. R. Procter perfected this method by packing a bell, similar in shape to a bottomless bottle of about 2 oz. (liq.) capacity, with the hide-powder, and siphoning the tan liquor up through the powder and over into a receiver. This deprives the tan liquor of tannin, and a portion of this non-tannin solution is evaporated to dryness and weighed till constant; similarly a portion of the original solution containing non-tannins and tannins is evaporated and weighed till constant; then the weight of the non-tannins subtracted from the weight of the non-tannins and tannins gives the weight of tannin, which is calculated to percentage on original solutions. This method was adopted as official by the International Association of Leather Trades Chemists until September 1906, when its faults were vividly brought before them by Gordon Parker of London and Bennett of Leeds, working in collaboration, although other but not so complete work had been previously done to the same end. The main faults of the method were that the hide-powder absorbed non-tannins, and therefore registered them as tannins, and the hide-powder was partially soluble. This difficulty has now been overcome to a large extent in the present official method of the I.A.L.T.C.
Meanwhile, Parker and Munro Payne proposed a new method of analysis, the essence of which is as follows:—A definite excess of lime solution is added to a definite quantity of tannin solution and the excess of lime estimated; the tan solution is now deprived of tannin by means of a soluble modification of gelatin, called “collin,” and the process is repeated. Thus we get two sets of figures, viz. total absorption and acid absorption (i.e.acids other than tan); the latter subtracted from the former gives tannin absorption, and this is calculated out in percentage of original liquor. The method failed theoretically, because a definite molecular weight had to be assumed for tannins which are all different. There are also several other objections, but though, like the hide-powder method, it is quite empirical, it gives exceedingly useful results if the rules for working are strictly adhered to.
The present official method of the I.A.L.T.C. is a modification of the American official method, which is in turn a modification of a method proposed by W. Eitner, of the Vienna Leather Research Station. The hide-powder is very slightly chrome-tanned with a basic solution of chromium chloride, 2 grammes of the latter being used per 100 grammes of hide-powder, and is then washed free from soluble salts and squeezed to contain 70% of moisture, and is ready for use. This preliminary chroming does away with the difficulty of the powder being soluble, by rendering it quite insoluble; it also lessens the tendency to absorb non-tannins. Such a quantity of this wet powder as contains 6.5 grammes of dry hide is now taken, and water is added until this quantity contains exactly 20 grammes of moisture,i.e.26.5 grammes in all; it is then agitated for 15 minutes with 100 c.c. of the prepared tannin solution, which is made up to contain tannin within certain definite limits, in a mechanical rotator, and filtered. Of this non-tannin solution 50 c.c. is then evaporated to dryness. The same thing is done with 50 c.c. of original solution containing non-tannins and tannins, and both residues are weighed. The tannin is thus determined by difference. The method does all that science can do at present. The rules for carrying out the analysis are necessarily very strict. The object in view is that all chemists should get exactly concordant results, and in this the I.A.L.T.C. has succeeded.
The work done by Wood, Trotman, Procter, Parker and others on the alkaloidal precipitation of tannin deserves mention.
Heavy Leathers.—The hides of oxen are received in the tanyard in four different conditions: (1) market or slaughter hides, which, coming direct from the local abattoirs, are soft, moist and covered with dirt and blood; (2) wet salted hides; (3) dry salted hides; (4) sun-dried or “flint” hides—the last three forms being the condition in which the imports of foreign hides are made. The first operation in the tannery is to clean the hides and bring them back as nearly as possible to the flaccid condition in which they left the animal’s back. The blood and other matter on market hides must be removed as quickly as possible, the blood being of itself a cause of dark stains and bad grain, and with the other refuse a source of putrefaction. When the hides are sound they are given perhaps two changes of water.