Copper Beech
[Illustration: FIG. 15.—Copper Beech. 1. Branch in winter state:a, leaf-scar;b, bud-scar. 2. Branch, with leaf-buds expanding, showing the plicate folding of the leaves.]
The leaf-scars are small, soon becoming merely ridges running half round the stem.
The bud-rings are very plain and easily counted. For this reason, and because it branches freely, it is a good tree for measurements of growth, as is seen in the following tables. Nos. 1, 2, 3 and 4: were made by a class of girls, from fourteen to sixteen, from a tree on my lawn. No. 5 was made by a pupil, whom I taught by correspondence, from a tree of the same species in another town. No. 6 was made by myself from my own tree. The measurements of the first four tables were somewhat revised by me, as they were not perfectly accurate. The pupils should always be cautioned to measure from the beginning of one set of rings to the beginning of the next.[1]
NO. 1.
NO. 2.
NO. 3.
NO. 4.
NO. 4. (cont.)
NO. 5.
NO. 6.
One question brought up by these measurements is whether there is any correspondence in growth between the main axis and its branches. It appears in these tables that there is a general correspondence, in this tree at least. In the recitation of the class, whose tables are given above (Nos. 1, 2, 3 and 4), we took all the measurements of these four branches for the year 1885 and added them. We did the same for 1886, and compared the results. The total growth for 1885 was 31-15/16 inches; for 1886, leaving out the measurement of the twig whose entire growth was in that year, 109-3/4 inches or nearly 3-1/2 times as much. The proportion held in a general way throughout, there being only a single case of a branch where the growth was greater in the first year.[1] But there is a point that must not be overlooked in this connection. The branches of the Beech seem to grow about equally well in the first, second, third, or any succeeding year. In some trees, as the Ash, the axillary buds make a large growth, and the succeeding terminal buds carry on the branch much more slowly; in other trees, as the Cherry, a branch grows very slowly in the first few years and then suddenly takes a start. These facts would appear in tables of growth, made from branches of these trees, but the addition of results for any particular year would have no significance.
In table No. 5, the addition of the measurements for 1885 and 1886 shows the growth in the latter year to be about twice that of the former. This branch came from a tree in another town. We have tried also to discover whether the number of leaves each year has any relation to growth. I cannot see that it has, but it requires many experiments to determine these points. To study this, make tables of the number of leaves on the branch each year. I think teachers would find it interesting to keep all data of this kind of work done by their classes, with a view to tabulation and comparison. The scholars themselves are exceedingly interested in anything that partakes of the nature of an original investigation.[1]
The leaf-arrangement of the Beech is alternate, on the one-half plan. The small twigs turn upwards, so that all the spray is on the upper side, giving a flat appearance to the branch.[1] This gives the leaves a better exposure to the light. Both the terminal and axillary buds grow freely, thus forming long, straight limbs, with many branches and much fine spray.
The bark of the Beech is beautifully smooth. The extreme straightness of the trunk and limbs is very striking, and may be compared to the crooked limbs of the Horsechestnut, where the branch is continually interrupted by the flower-cluster. In the Beech the flowers are axillary.
QUESTIONS ON THE BEECH.
How are the scales of the Beech bud arranged?
How many leaves are there in the bud?
How does the arrangement of the scales and leaves in the bud differ from that of the Horsechestnut?
How are the leaves folded in the bud?
What is the arrangement of the leaves on the stem?
How does this differ from Horsechestnut and Lilac?
How old is your branch?
How old is each twig?
What years were the best for growth?
How does the growth of the branches differ from that of Horsechestnut? From Lilac?
Explain these differences with reference to the growth and arrangement of the buds?
In what direction do the twigs grow?
How does this affect the appearance of the tree?
Compare the amount of spray of the Beech and Horsechestnut and explain the reason of the difference.
These questions are only intended for review, they are never to be used for the first study of the specimen.
AMERICAN ELM (Ulmus Americana).
The buds are covered with brown scales, which are hairy on the edges. The flower-buds are larger than the leaf-buds and are in the axils of the lower leaves of the preceding year. Each leaf in the bud is enclosed by a pair of scales. They are so small that the pupils, unused to delicate work, will hardly discover them. Under a glass they can be seen to be ovate, folded on the midrib with the inner face within (conduplicate), and with an ovate scale joined to the base of the leaf on either side. The scales thus show themselves to be modified stipules. The venation of the leaves is very plain. The scales are much larger than the leaves. The flower-buds contain a cluster of flowers, on slender green pedicels. The calyx is bell-shaped, unequal, and lobed. The stamens and pistil can be seen. The flower-clusters do not seem to leave any mark which is distinguishable from the leaf-scar.
American Elm
[Illustration: FIG. 16.—American Elm. 1. Branch in winter state:a, leaf-scars;b, bud-scars;d, leaf-buds;e, flower-buds. 2. Branch, with staminate flower-buds expanding. 3. Same, more advanced. 4. Branch, with pistillate flowers, the leaf-bud also expanding. ]
The leaf-scars are small and extend about half around the stem. The arrangement is alternate on the one-half plan. There are three dots on the scar.
The rings are quite plain. The tree can be used to make tables of growth, like those of the Beech.
The buds will probably be too small for examination by the pupils, at present, but their position and development can be studied, and are very instructive. As the leaf-buds are all on the ends of the branchlets, the twigs and branches will be just below the bud-rings, and then there will be a space where no twigs nor branches will be found, till the next set of rings is reached. This gives the branches more room to develop symmetrically. The terminal buds do not develop in the Elm, in old trees, the bud axillary to the last leaf of the season taking its place, and most of the other axillary buds growing also. This makes the tree break out into very fine spray. A tree like the Elm, where the trunk becomes lost in the branches, is calleddeliquescent; when the trunk is continued to the top of the tree, as in the Spruce, it isexcurrent.
The small, feathery twigs and branches that are often seen on the trunks and great limbs of the elm grow from buds which are produced anywhere on the surface of the wood. Such buds are calledadventitiousbuds. They often spring from a tree when it is wounded.
"The American elm is, in most parts of the state, the most magnificent tree to be seen. From a root, which, in old trees, spreads much above the surface of the ground, the trunk rises to a considerable height in a single stem. Here it usually divides into two or three principal branches, which go off by a gradual and easy curve. Theses stretch upwards and outwards with an airy sweep, become horizontal, the extreme half of the limb, pendent, forming a light and regular arch. This graceful curvature, and absence of all abruptness, in the primary limbs and forks, and all the subsequent divisions, are entirely characteristic of the tree, and enable an observer to distinguish it in the winter and even by night, when standing in relief against the sky, as far as it can be distinctly seen."[1]
QUESTIONS ON THE AMERICAN ELM.
How do the flower-buds differ from the leaf-buds in position and appearance?
What is the arrangement of the leaves?
What other tree that you have studied has this arrangement?
How old is your branch?
Where would you look to see if the flower-cluster had left any mark?
Why is it that several twigs grow near each other, and that then comes a space without any branches?
What buds develop most frequently?
How does this affect the appearance of the tree?
What is a tree called when the trunk is lost in the branches?
BALM OF GILEAD (Populus balsamifera, var. candicans).
The buds are pointed: the terminal slightly angled, the axillary flattened against the stem.[1] Some of the axillary buds contain leaves and some flowers; the appearance of the leaf-buds and flower-buds being the same. The scales of the bud are modified stipules. The terminal buds have about three pairs of the outer scales brown and leathery. The inner scales, as well as the leaves, are coated with resinous matter, which has a strong odor and a nauseous taste. The smaller outer scales have no corresponding leaf, and apparently are modified stipules of the leaves of the preceding year, but the larger ones have a leaf to each pair of scales. The outer and inner leaves are small, the middle ones larger. Comparing the branch, it will be seen that these leaves make the largest growth of internode. The leaves are rolled towards the midrib on the upper face (involute). There are about ten which are easily seen and counted, the inner ones being very small, with minute scales. The axillary buds have a short thick scale on the outer part of the bud, then about three pairs of large scales, each succeeding one enwrapping those within, the outer one brown and leathery. The scales of the flower-buds are somewhat gummy, but not nearly so much so as those of the leaf-buds. Within is the catkin. Each pistil, or stamen (they are on separate trees,dioecious) is in a little cup and covered by a scale, which is cut and fringed.
The leaf-scars are somewhat three-lobed on the young parts, with three dots, indicating the fibro-vascular bundles, which ran up into the leaf. The scars are swollen, making the young branches exceedingly rough. In the older parts the scars become less noticeable. Strong young shoots, especially those which come up from the root, are strongly angled, with three ridges running up into each leaf-scar, making them almost club-shaped. There are often from twenty to thirty leaves in one year's growth, in such shoots, and all the leaves are not rudimentary in the bud. The growth in this case is said to beindefinite. Usually in trees with scaly buds the plan of the whole year's growth is laid down in the bud, and the termdefiniteis applied. Branches, like the Rose, that go on growing all summer grow indefinitely.
The bud-scale scar is quite different from the other trees which we have examined. It is not composed of definite rings, but of leaf-scars with long ridges running from each side of them, showing the scales to be modified stipules. The leaf-scars have become somewhat separated by the growth of the internodes. In the Beech, there are eight, or more, pairs of scales with no leaves, so that the internodes do not develop, and a ring is left on the branch.
The flower-cluster leaves a concave, semicircular scar, in the leaf-axil.
Balm-of-Gilead
[Illustration: FIG. 17.—Balm-of-Gilead. 1. Branch in winter state:a, leaf-scar;b, bud-scar. 2. Branch, with leaf-buds expanded. 3. Branch, with catkin appearing from the bud.]
The terminal buds are the strongest and not very many axillary buds develop, so that the tree has not fine spray.
The leaf-arrangement is alternate, on the 2/5 plan. Phyllotaxy is not yet to be taken up, but the pupils should be shown the different angles of the branching of the twigs, and told to compare them with Beech and Elm.
QUESTIONS ON THE BALM OF GILEAD.
In which buds are the flower-clusters?
Are there flowers and leaves in the same buds?
What are the scales of the bud?
How are the leaves folded in the bud?
How do the axillary and terminal buds differ?
What are the dots on the leaf-scars?
Why is there no distinct band of rings as in Beech?
How old is your branch?
Where do you look for flower-cluster scars?
Which buds are the strongest?
How does this affect the appearance of the tree?
What makes the ends of the branches so rough?
Compare the arrangement of the twigs and branches with Beech and Elm, with Horsechestnut and Lilac.
TULIP-TREE (Liriodendron Tulipifera).
The buds are small, flat, and rounded at the apex. They are sheathed by scales, each leaf being covered by a pair, whose edges cohere. The outer pair are brown and are the stipules of the last leaf of the preceding year. The leaves are conduplicate, as in Magnolia, and have the blade bent inwards on the petiole (inflexed). Their shape is very clearly to be seen, and no bud is more interesting in the closeness of its packing. Axillary buds are often found within. The flowers grow high upon the trees and towards the ends of the branches.
The leaf-scars are round with many dots. The scar of the stipules is a continuous line around the stem, as in Magnolia.
CHERRY(Prunus Cerasus).
The leaf-buds are terminal, or in the axils of the upper leaves of the preceding year; the flower buds are axillary. There is but one bud in each axil, and usually two or three flowers in each bud, but the leaves on the twigs are crowded and the flowers therefore appear in clusters. The blossom-buds are larger and more rounded than the leaf-buds.
The buds of the tree develop very easily in the house, and as they are so small they can be better studied in watching them come out, than by attempting to dissect them, unless the scholars are sufficiently advanced to use the microscope easily. It is always bad for a pupil to attempt to describe what he sees but imperfectly. He will be sure to jump at any conclusions which he thinks ought to be correct.
The leaf-scars are semicircular, small and swollen.
The bud-rings are plain. The twigs make a very small growth in a season, so that the leaf-scars and rings make them exceedingly rough.
The flower-cluster scars are small circles, with a dot in the centre, in the leaf-axils. The flowers come before the leaves.
The leaf-arrangement is alternate on the 2/5 plan. The pupils may compare the branching with that of their other specimens.
RED MAPLE (Acer rubrum).
This is a good specimen for the study of accessory buds. There is usually a bud in the axil of each lower scale of the axillary buds, making three side by side. We have already noticed this as occurring sometimes in Lilac. It is habitually the case with the Red Maple. The middle bud, which is smaller and develops later, is a leaf-bud. The others are flower-buds.
The leaf-scars are small, with three dots on each scar. The rings are very plain. The flower-cluster leaves a round scar in the leaf-axil, as in Cherry.
The leaves are opposite and the tree branches freely. The twigs seem to be found just below the bud-rings, as the upper leaf-buds usually develop best and the lower buds are single, containing flowers only.
NORWAY SPRUCE (Picea excelsa).
The buds are terminal, and axillary, from the axils of the leaves of the preceding year, usually from those at the ends of the branchlets. They are covered with brown scales and contain many leaves.
Branch of Cherry
[Illustration: FIG. 18.—Branch of Cherry in winter state:a, leaf-scar;b, bud-scar;c, flower-scar.]
[Illustration: FIG. 19.—Branch of Red Maple in winter state (reduced). 2. Flower-buds]
The leaves are needle-shaped and short.[1] They are arranged densely on the branches, alternately on the 8/21 plan (see section on phyllotaxy). When they drop off they leave a hard, blunt projection which makes the stem very rough. As the terminal bud always develops unless injured, the tree is excurrent, forming a straight trunk, throwing out branches on every side. The axillary buds develop near the ends of the branchlets, forming apparent whorls of branches around the trunk. In the smaller branches, as the tree grows older, the tendency is for only two buds to develop nearly opposite each other, forming a symmetrical branch.
The bud-scales are persistent on the branches and the growth from year to year can be traced a long way back.
The cones hang on the ends of the upper branches. They are much larger than in our native species of Black and White Spruce.
The Evergreens are a very interesting study and an excellent exercise in morphology for the older scholars.
2.Vernation. This term signifies the disposition of leaves in the bud, either in respect to the way in which each leaf is folded, or to the manner in which the leaves are arranged with reference to each other. The pupils have described the folding of the leaves in some of their specimens.
In the Beech, the leaf isplicate, or plaited on the veins. In the Elm, Magnolia, and Tulip-tree, it isconduplicate, that is, folded on the midrib with the inner face within. In the Tulip-tree, it is alsoinflexed, the blade bent forwards on the petiole. In the Balm of Gilead, the leaf isinvolute, rolled towards the midrib on the upper face.
Other kinds of vernation arerevolute, the opposite of involute, where the leaf is rolled backwards towards the midrib;circinate, rolled from the apex downwards, as we see in ferns; andcorrugate, when the leaf is crumpled in the bud.
Branch of Norway Spruce
[Illustration: FIG. 20.—Branch of Norway Spruce.]
In all the trees we have studied, the leaves simply succeed each other, each leaf, or pair of leaves, overlapping the next in order. The names of the overlapping of the leaves among themselves,imbricated, convolute, etc., will not be treated here, as they are not needed. They will come underæstivation, the term used to describe the overlapping of the modified leaves, which make up the flower.[1]
3.Phyllotaxy. The subject of leaf-arrangement is an extremely difficult one, and it is best, even with the older pupils, to touch it lightly. The point to be especially brought out is the disposition of the leaves so that each can get the benefit of the light. This can be seen in any plant and there are many ways in which the desired result is brought about. The chief way is the distribution of the leaves about the stem, and this is well studied from the leaf-scars.
The scholars should keep the branches they have studied. It is well to have them marked with the respective names, that the teacher may examine and return them without fear of mistakes.
In the various branches that the pupils have studied, they have seen that the arrangement of the leaves differs greatly. The arrangement of leaves is usually classed under three modes: thealternate, theopposite, and thewhorled; but the opposite is the simplest form of the whorled arrangement, the leaves being in circles of two. In this arrangement, the leaves of each whorl stand over the spaces of the whorl just below. The pupils have observed and noted this in Horsechestnut and Lilac. In these there are four vertical rows or ranks of leaves. In whorls of three leaves there would be six ranks, in whorls of four, eight, and so on.
When the leaves are alternate, or single at each node of the stem, they are arranged in many different ways. Ask the pupils to look at all the branches with alternate leaves that they have studied, and determine in each case what leaves stand directly over each other. That is, beginning with any leaf, count the number of leaves passed on the stem, till one is reached that stands directly over the first.[1] In the Beech and the Elm the leaves are on opposite sides of the stem, so that the third stands directly over the first. This makes two vertical ranks, or rows, of leaves, dividing the circle into halves. It is, therefore, called the 1/2 arrangement. Another way of expressing it is to say that the angular divergence between the leaves is 180°, or one-half the circumference.
The 1/3 arrangement, with the leaves in three vertical ranks, is not very common. It may be seen in Sedges, in the Orange-tree, and in Black Alder(Ilex verticillata). In this arrangement, there are three ranks of leaves, and each leaf diverges from the next at an angle of 120°, or one-third of the circumference.
By far the commonest arrangement is with the leaves in five vertical ranks. The Cherry, the Poplar, the Larch, the Oak, and many other trees exhibit this. In this arrangement there are five leaves necessary to complete the circle. We might expect, then, that each leaf would occupy one-fifth of the circle. This would be the case were it not for the fact that we have to pass twice around the stem in counting them, so that each leaf has twice as much room, or two-fifths of the circle, to itself. This is, therefore, the 2/5 arrangement. This can be shown by winding a thread around the stem, passing it over each leaf-scar. In the Beech we make one turn of the stem before reaching the third leaf which stands over the first. In the Apple the thread will wind twice about the stem, before coming to the sixth leaf, which is over the first.
Another arrangement, not very common, is found in the Magnolia, the Holly, and the radical leaves of the common Plantain and Tobacco. The thread makes three turns of the stem before reaching the eighth leaf which stands over the first. This is the 3/8 arrangement. It is well seen in the Marguerite, a greenhouse plant which is very easily grown in the house.
Look now at these fractions, 1/2, 1/3, 2/5, and 3/8. The numerator of the third is the sum of the numerators of the first and second, its denominator, the sum of the two denominators. The same is true of the fourth fraction and the two immediately preceding it. Continuing the series, we get the fractions 5/13, 8/21, 13/34. These arrangements can be found in nature in cones, the scales of which are modified leaves and follow the laws of leaf-arrangement.[1]
[1]"It is to be noted that the distichous or 1/2 variety gives the maximum divergence, namely 180°, and that the tristichous, or 1/3, gives the least, or 120°; that the pentastichous, or 2/5, is nearly the mean between the first two; that of the 3/8, nearly the mean between the two preceding, etc. The disadvantage of the two-ranked arrangement is that the leaves are soon superposed and so overshadow each other. This is commonly obviated by the length of the internodes, which is apt to be much greater in this than in the more complex arrangements, therefore placing them vertically further apart; or else, as in Elms, Beeches, and the like, the branchlets take a horizontal position and the petioles a quarter twist, which gives full exposure of the upper face of all the leaves to the light. The 1/3 and 2/5, with diminished divergence, increase the number of ranks; the 3/8 and all beyond, with mean divergence of successive leaves, effect a more thorough distribution, but with less and less angular distance between the vertical ranks."
For directions for finding the arrangement of cones, see Gray's Structural Botany, Chap. IV, Sect. 1.
The subject appears easy when stated in a text-book, but, practically, it is often exceedingly difficult to determine the arrangement. Stems often twist so as to alter entirely the apparent disposition of the leaves. The general principle, however, that the leaves are disposed so as to get the best exposure to air and light is clear. This cannot be shown by the study of the naked branches merely, because these do not show the beautiful result of the distribution.[1] Many house plants can be found, which will afford excellent illustrations (Fig. 21). The Marguerite and Tobacco, both easily grown in the house, are on the 3/8 plan. The latter shows the eight ranks most plainly in the rosette of its lower leaves. The distribution is often brought about by differences in the lengths of the petioles, as in a Horsechestnut branch (Fig. 22) where the lower, larger leaves stand out further from the branch than the upper ones; or by a twist in the petioles, so that the upper faces of the leaves are turned up to the light, as in Beech (Fig. 23). If it is springtime when the lessons are given, endless adaptations can be found.
Branch of Geranium
[Illustration: FIG. 21. Branch of Geranium, viewed from above.]
Figure 22a
Figure 22b
[Illustration: FIG. 22.]
Figure 23
[Illustration: FIG. 23.]
Gray's First Lessons. Sect. IV. VII, §4.How Plants Grow. Chap. I, 51-62; I, 153.
The stem, as the scholars have already learned, is the axis of the plant. The leaves are produced at certain definite points called nodes, and the portions of stem between these points are internodes. The internode, node, and leaf make a single plant-part, and the plant is made up of a succession of such parts.
The stem, as well as the root and leaves, may bear plant-hairs. The accepted theory of plant structure assumes that these four parts, root, stem, leaves, and plant-hairs, are the only members of a flowering plant, and that all other forms, as flowers, tendrils, etc., are modified from these. While this idea is at the foundation of all our teaching, causing us to lead the pupil to recognize as modified leaves the cotyledons of a seedling and the scales of a bud, it is difficult to state it directly so as to be understood, except by mature minds. I have been frequently surprised at the failure of even bright and advanced pupils to grasp this idea, and believe it is better to let them first imbibe it unconsciously in their study. Whenever their minds are ready for it, it will be readily understood. The chief difficulty is that they imagine that there is a direct metamorphosis of a leaf to a petal or a stamen.
Briefly, the theory is this: the beginnings of leaf, petal, tendril, etc., are the same. At an early stage of their growth it is impossible to tell what they are to become. They develop into the organ needed for the particular work required of them to do. The organ, that under other circumstances might develop into a leaf, is capable of developing into a petal, a stamen, or a pistil, according to the requirements of the plant, but no actual metamorphosis takes place. Sometimes, instead of developing into the form we should normally find, the organ develops into another form, as when a petal stands in the place of a stamen, or the pistil reverts to a leafy branch. This will be more fully treated under flowers. The study of the different forms in which an organ may appear is the study ofmorphology.
1.Forms of Stems.—Stems may grow in many ways. Let the pupils compare the habits of growth of the seedlings they have studied. The Sunflower and Corn areerect. This is the most usual habit, as with our common trees. The Morning Glory istwining, the stem itself twists about a support. The Bean, Pea and Nasturtium areclimbing. The stems are weak, and are held up, in the first two by tendrils, in the last by the twining leaf-stalks. The English Ivy, as we have seen, is also climbing, by means of its aërial roots. The Red Clover isascending, the branches rising obliquely from the base. Some kinds of Clover, as the White Clover, arecreeping, that is, with prostrate branches rooting at the nodes and forming new plants. Such rooting branches are calledstolons, or when the stem runs underground,suckers. The gardener imitates them in the process called layering, that is, bending down an erect branch and covering it with soil, causing it to strike root. When the connecting stem is cut, a new plant is formed. Long and leafless stolons, like those of the Strawberry are calledrunners. Stems creep below the ground as well as above. Probably the pupil will think of some examples. The pretty little Gold Thread is so named from the yellow running stems, which grow beneath the ground and send up shoots, or suckers, which make new plants. Many grasses propagate themselves in this way. Such stems are calledrootstocks. "That these are really stems, and not roots, is evident from the way in which they grow; from their consisting of a succession of joints; and from the leaves which they bear on each node, in the form of small scales, just like the lowest ones on the upright stem next the ground. They also produce buds in the axils of these scales, showing the scales to be leaves; whereas real roots bear neither leaves nor axillary buds."[1] Rootstocks are often stored with nourishment. We have already taken up this subject in the potato, but it is well to repeat the distinction between stems and roots. A thick, short rootstock provided with buds, like the potato, is called atuber. Compare again the corm of Crocus and the bulb of Onion to find the stem in each. In the former, it makes the bulk of the whole; in the latter, it is a mere plate holding the fleshy bases of the leaves.
2.Movements of Stems.—Let a glass thread, no larger than a coarse hair, be affixed by means of some quickly drying varnish to the tip of the laterally inclined stem of one of the young Morning-Glory plants in the schoolroom. Stand a piece of cardboard beside the pot, at right angles to the stem, so that the end of the glass will be near the surface of the card. Make a dot upon the card opposite the tip of the filament, taking care not to disturb the position of either. In a few minutes observe that the filament is no longer opposite the dot. Mark its position anew, and continue thus until a circle is completed on the cardboard. This is a rough way of conducting the experiment. Darwin's method will be found in the footnote.[1]
The use of the glass filament is simply to increase the size of the circle described, and thus make visible the movements of the stem. All young parts of stems are continually moving in circles or ellipses. "To learn how the sweeps are made, one has only to mark a line of dots along the upper side of the outstretched revolving end of such a stem, and to note that when it has moved round a quarter of a circle, these dots will be on one side; when half round, the dots occupy the lower side; and when the revolution is completed, they are again on the upper side. That is, the stem revolves by bowing itself over to one side,—is either pulled over or pushed over, or both, by some internal force, which acts in turn all round the stem in the direction in which it sweeps; and so the stem makes its circuits without twisting."[1]
The nature of the movement is thus a successive nodding to all the points of the compass, whence it is called by Darwincircumnutation. The movement belongs to all young growing parts of plants. The great sweeps of a twining stem, like that of the Morning-Glory, are only an increase in the size of the circle or ellipse described.[1]
When a young stem of a Morning-Glory, thus revolving, comes in contact with a support, it will twist around it, unless the surface is too smooth to present any resistance to the movement of the plant. Try to make it twine up a glass rod. It will slip up the rod and fall off. The Morning-Glory and most twiners move around from left to right like the hands of a clock, but a few turn from right to left.
While this subject is under consideration, the tendrils of the Pea and Bean and the twining petioles of the Nasturtium will be interesting for comparison. The movements can be made visible by the same method as was used for the stem of the Morning-Glory. Tendrils and leaf petioles are often sensitive to the touch. If a young leaf stalk of Clematis be rubbed for a few moments, especially on the under side, it will be found in a day or two to be turned inward, and the tendrils of the Cucumber vine will coil in a few minutes after being thus irritated.[1] The movements of tendrils are charmingly described in the chapter entitled "How Plants Climb," in the little treatise by Dr. Gray, already mentioned.
The so-called "sleep of plants" is another similar movement. The Oxalis is a good example. The leaves droop and close together at night, protecting them from being chilled by too great radiation.
The cause of these movements is believed to lie in changes of tension preceding growth in the tissues of the stem.[1] Every stem is in a state of constant tension. Naudin has thus expressed it, "the interior of every stem is too large for its Jacket."[2] If a leaf-stalk of Nasturtium be slit vertically for an inch or two, the two halves will spring back abruptly. This is because the outer tissues of the stem are stretched, and spring back like india-rubber when released. If two stalks twining in opposite directions be slit as above described, the side of the stem towards which each stalk is bent will spring back more than the other, showing the tension to be greater on that side. A familiar illustration of this tension will be found in the Dandelion curls of our childhood.
The movements of the Sensitive Plant are always very interesting to pupils, and it is said not to be difficult to raise the plants in the schoolroom. The whole subject, indeed, is one of the most fascinating that can be found, and its literature is available, both for students and teachers. Darwin's essay on "Climbing Plants," and his later work on the "Power of Movement in Plants," Dr. Gray's "How Plants Behave," and the chapter on "Movements" in the "Physiological Botany," will offer a wide field for study and experiment.
3.Structure of Stems.—Let the pupils collect a series of branches of some common tree or shrub, from the youngest twig up to as large a branch as they can cut, and describe them. Poplar, Elm, Oak, Lilac, etc., will be found excellent for the purpose.
While discussing these descriptions, a brief explanation of plant-structure may be given. In treating this subject, the teacher must govern himself by the needs of his class, and the means at his command. Explanations requiring the use of a compound microscope do not enter necessarily into these lessons. The object aimed at is to teach the pupils about the things which they can see and handle for themselves. Looking at sections that others have prepared is like looking at pictures; and, while useful in opening their eyes and minds to the wonders hidden from our unassisted sight, fails to give the real benefit of scientific training. Plants are built up of cells. The delicate-walled spherical, or polygonal, cells which make up the bulk of an herbaceous stem, constitute cellular tissue (parenchyma). This was well seen in the stem of the cutting of Bean in which the roots had begun to form.[1] The strengthening fabric in almost all flowering plants is made up of woody bundles, or woody tissue.[2] The wood-cells are cells which are elongated and with thickened walls. There are many kinds of them. Those where the walls are very thick and the cavity within extremely small arefibres. A kind of cell, not strictly woody, is where many cells form long vessels by the breaking away of the connecting walls. These areducts. These two kinds of cells are generally associated together in woody bundles, called therefore fibro-vascular bundles. We have already spoken of them as making the dots on the leaf-scars, and forming the strengthening fabric of the leaves.[3]
We will now examine our series of branches. The youngest twigs, in spring or early summer, are covered with a delicate, nearly colorless skin. Beneath this is a layer of bark, usually green, which gives the color to the stem, an inner layer of bark, the wood and the pith. The pith is soft, spongy and somewhat sappy. There is also sap between the bark and the wood. An older twig has changed its color. There is a layer of brown bark, which has replaced the colorless skin. In a twig a year old the wood is thicker and the pith is dryer. Comparing sections of older branches with these twigs, we find that the pith has shrunk and become quite dry, and that the wood is in rings. It is not practicable for the pupils to compare the number of these rings with the bud-rings, and so find out for themselves that the age of the branch can be determined from the wood, for in young stems the successive layers are not generally distinct. But, in all the specimens, the sap is found just between the wood and the bark, and here, where the supply of food is, is where the growth is taking place. Each year new wood and new bark are formed in thiscambium-layer, as it is called, new wood on its inner, new bark on its outer face. Trees which thus form a new ring of wood every year are calledexogenous, or outside-growing.
Ask the pupils to separate the bark into its three layers and to try the strength of each. The two outer will easily break, but the inner is generally tough and flexible. It is this inner bark, which makes the Poplar and Willow branches so hard to break. These strong, woody fibres of the inner bark give us many of our textile fabrics. Flax and Hemp come from the inner bark of their respective plants (Linum usitatissimumandCannabis sativa), and Russia matting is made from the bark of the Linden (Tilia Americana).
We have found, in comparing the bark of specimens of branches of various ages, that, in the youngest stems, the whole is covered with a skin, orepidermis, which is soon replaced by a brown outer layer of bark, called thecorky layer; the latter gives the distinctive color to the tree. While this grows, it increases by a living layer of cork-cambium on its inner face, but it usually dies after a few years. In some trees it goes on growing for many years. It forms the layers of bark in the Paper Birch and the cork of commerce is taken from the Cork Oak of Spain. The green bark is of cellular tissue, with some green coloring matter like that of the leaves; it is at first the outer layer, but soon becomes covered with cork. It does not usually grow after the first year. Scraping the bark of an old tree, we find the bark homogeneous. The outer layers have perished and been cast off. As the tree grows from within, the bark is stretched and, if not replaced, cracks and falls away piecemeal. So, in most old trees, the bark consists of successive layers of the inner woody bark.
Stems can be well studied from pieces of wood from the woodpile. The ends of the log will show the concentric rings. These can be traced as long, wavy lines in vertical sections of the log, especially if the surface is smooth. If the pupils can whittle off different planes for themselves, they will form a good idea of the formation of the wood. In many of the specimens there will be knots, and the nature of these will be an interesting subject for questions. If the knot is near the centre of the log, lead back their thoughts to the time when the tree was as small as the annular ring on which the centre of the knot lies. Draw a line on this ring to represent the tree at this period of its growth. What could the knot have been? It has concentric circles like the tree itself. It was a branch which decayed, or was cut off. Year after year, new rings of wood formed themselves round this broken branch, till it was covered from sight, and every year left it more deeply buried in the trunk.
Extremely interesting material for the study of wood will be found in thin sections prepared for veneers. Packages of such sections will be of great use to the teacher.[1] They show well the reason of the formation of a dividing line between the wood of successive seasons. In a cross section of Oak or Chestnut the wood is first very open and porous and then close. This is owing to the presence of ducts in the wood formed in the spring. In other woods there are no ducts, or they are evenly distributed, but the transition from the close autumn wood, consisting of smaller and more closely packed cells, to the wood of looser texture, formed in the following spring, makes a line that marks the season's growth.
Let each of the scholars take one of the sections of Oak and write a description of its markings. The age is easily determined; the pith rays, ormedullary rays, are also plain. These form what is called the silver grain of the wood. The ducts, also, are clear in the Oak and Chestnut. There is a difference in color between the outer and inner wood, the older wood becomes darker and is called theheart-wood, the outer is thesap-wood. In Birds-eye Maple, and some other woods, the abortive buds are seen. They are buried in the wood, and make the disturbance which produces the ornamental grain. In sections of Pine or Spruce, no ducts can be found. The wood consists entirely of elongated, thickened cells or fibres. In some of the trees the pith rays cannot be seen with the naked eye.
Let the pupils compare the branches which they have described, with a stalk of Asparagus, Rattan, or Lily. A cross section of one of these shows dots among the soft tissue. These are ends of the fibro-vascular bundles, which in these plants are scattered through the cellular tissue instead of being brought together in a cylinder outside of the pith. In a vertical section they appear as lines. There are no annular rings.
If possible, let the pupils compare the leaves belonging to these different types of stems. The parallel-veined leaves of monocotyledons have stems without distinction of wood, bark and pith; the netted-veined leaves of dicotyledons have exogenous stems.
Dicotyledons have bark, wood, and pith, and grow by producing a new ring of wood outside the old. They also increase by the growth of the woody bundles of the leaves, which mingle with those of the stem.[1] Twist off the leaf-stalk of any leaf, and trace the bundles into the stem.
Monocotyledons have no layer which has the power of producing new wood, and their growth takes place entirely from the intercalation of new bundles, which originate at the bases of the leaves. The lower part of a stem of a Palm, for instance, does not increase in size after it has lost its crown of leaves. This is carried up gradually. The upper part of the stem is a cone, having fronds, and below this cone the stem does not increase in diameter. The wordendogenous, inside-growing, is not, therefore, a correct one to describe the growth of most monocotyledons, for the growth takes place where the leaves originate, near the exterior of the stem.
Gray's First Lessons. Sect. VI. Sect, XVI, §1, 401-13. §3. §6, 465-74.
How Plants Grow. Chap. 1, 82, 90-118.