Authorities.—For the history of lead see W. H. Pulsifer,Notes for a History of Lead(1888); B. Neumann,Die Metalle(1904); A. Rossing,Geschichte der Metalle(1901). For the chemistry see H. Roscoe and C. Schorlemmer,Treatise on Inorganic Chemistry, vol. ii. (1897); H. Moissan,Traité de chimie minerale; O. Dammer,Handbuch der anorganischen Chemie. For the metallurgy see J. Percy,The Metallurgy of Lead(London, 1870); H. F. Collins,The Metallurgy of Lead and Silver(London, 1899), part i. “Lead”; H. O. Hofmann,The Metallurgy of Lead(6th ed., New York, 1901); W. R. Ingalls,Lead Smelting and Refining(1906); A. G. Betts,Lead Refining by Electrolysis(1908); M. Eissler,The Metallurgy of Argentiferous Silver.The Mineral Industry, begun in 1892, annually records the progress made in lead smelting.
Authorities.—For the history of lead see W. H. Pulsifer,Notes for a History of Lead(1888); B. Neumann,Die Metalle(1904); A. Rossing,Geschichte der Metalle(1901). For the chemistry see H. Roscoe and C. Schorlemmer,Treatise on Inorganic Chemistry, vol. ii. (1897); H. Moissan,Traité de chimie minerale; O. Dammer,Handbuch der anorganischen Chemie. For the metallurgy see J. Percy,The Metallurgy of Lead(London, 1870); H. F. Collins,The Metallurgy of Lead and Silver(London, 1899), part i. “Lead”; H. O. Hofmann,The Metallurgy of Lead(6th ed., New York, 1901); W. R. Ingalls,Lead Smelting and Refining(1906); A. G. Betts,Lead Refining by Electrolysis(1908); M. Eissler,The Metallurgy of Argentiferous Silver.The Mineral Industry, begun in 1892, annually records the progress made in lead smelting.
LEADER, BENJAMIN WILLIAMS(1831- ), English painter, the son of E. Leader Williams, an engineer, received his art education first at the Worcester School of Design and later in the schools of the Royal Academy. He began to exhibit at the Academy in 1854, was elected A.R.A. in 1883 and R.A. in 1898, and became exceedingly popular as a painter of landscape. His subjects are attractive and skilfully composed. He was awarded a gold medal at the Paris Exhibition in 1889, and was made a knight of the Legion of Honour. One of his pictures, “The Valley of the Llugwy,” is in the National Gallery of British Art.
SeeThe Life and Work of B. W. Leader, R.A., by Lewis Lusk,Art JournalOffice (1901).
SeeThe Life and Work of B. W. Leader, R.A., by Lewis Lusk,Art JournalOffice (1901).
LEADHILLITE,a rare mineral consisting of basic lead sulphato-carbonate, Pb4SO4(CO3)2(OH)2. Crystals have usually the form of six-sided plates (fig. 1) or sometimes of acute rhombohedra (fig. 2); they have a perfect basal cleavage (parallel to P in fig. 1) on which the lustre is strongly pearly; they are usually white and translucent. The hardness is 2.5 and the sp. gr. 6.26-6.44. The crystallographic and optical characters point to the existence of three distinct kinds of leadhillite, which are, however, identical in external appearance and may even occur intergrown together in the same crystal: (a) monoclinic with an optic axial angle of 20°; (b) rhombohedral (fig. 2) and optically uniaxial; (c) orthorhombic (fig. 1) with an optic axial angle of 72¾°. The first of these is the more common kind, and the second has long been known under the name susannite. The fact that the published analyses of leadhillite vary somewhat from the formula given above suggests that these three kinds may also be chemically distinct.
Leadhillite is a mineral of secondary origin, occurring with cerussite, anglesite, &c., in the oxidized portions of lead-bearing lodes; it has also been found in weathered lead slags left by the Romans. It has been found most abundantly in the Susanna mine at Leadhills in Scotland (hence the names leadhillite and susannite). Good crystals have also been found at Red Gill in Cumberland and at Granby in Missouri. Crystals from Sardinia have been called maxite.
(L. J. S.)
LEADHILLS,a village of Lanarkshire, Scotland, 5¾ m. W.S.W. of Elvanfoot station on the Caledonian Railway Company’s main line from Glasgow to the south. Pop. (1901) 835. It is the highest village in Scotland, lying 1301 ft. above sea-level, near the source of Glengonner Water, an affluent of the Clyde. It is served by a light railway. Lead and silver have been mined here and at Wanlockhead, 1½ m. S.W., for many centuries—according to some authorities even in Roman days. Gold was discovered in the reign of James IV., but though it is said then to have provided employment for 300 persons, its mining has long ceased to be profitable. The village is neat and well built, and contains a masonic hall and library, the latter founded by the miners about the middle of the 18th century. Allan Ramsay, the poet, and William Symington (1763-1831), one of the earliest adaptors of the steam engine to the purposes of navigation, were born at Leadhills.
LEAD POISONING,orPlumbism, a “disease of occupations,” which is itself the cause of organic disease, particularly of the nervous and urinary systems. The workpeople affected are principally those engaged in potteries where lead-glaze is used; but other industries in which health is similarly affected are file-making, house-painting and glazing, glass-making, copper-working, coach-making, plumbing and gasfitting, printing, cutlery, and generally those occupations in which lead is concerned.
The symptoms of chronic lead poisoning vary within very wide limits, from colic and constipation up to total blindness, paralysis, convulsions and death. They are thus described by Dr J. T. Arlidge (Diseases of Occupations):—
The poison finds its way gradually into the whole mass of the circulating blood, and exerts its effects mainly on the nervous system, paralysing nerve-force and with it muscular power. Its victims become of a sallow-waxy hue; the functions of the stomach and bowels are deranged, appetite fails and painful colic with constipation supervenes. The loss of power is generally shown first in the fingers, hands and wrists, and the condition known as “wrist-drop” soon follows, rendering the victim useless for work. The palsy will extend to the shoulders, and after no long time to the legs also. Other organs frequently involved are the kidneys, the tissue of which becomes permanently damaged; whilst the sight is weakened or even lost.
The poison finds its way gradually into the whole mass of the circulating blood, and exerts its effects mainly on the nervous system, paralysing nerve-force and with it muscular power. Its victims become of a sallow-waxy hue; the functions of the stomach and bowels are deranged, appetite fails and painful colic with constipation supervenes. The loss of power is generally shown first in the fingers, hands and wrists, and the condition known as “wrist-drop” soon follows, rendering the victim useless for work. The palsy will extend to the shoulders, and after no long time to the legs also. Other organs frequently involved are the kidneys, the tissue of which becomes permanently damaged; whilst the sight is weakened or even lost.
Dr M‘Aldowie, senior physician to the North Staffordshire Infirmary, has stated that “in the pottery trade lead is very slow in producing serious effects compared with certain other industries.” In his experience the average period of working in lead before serious lesions manifest themselves is 18 years for females and 22½ years for males. But some individuals fall victims to the worst forms of plumbism after a few months’ or even weeks’ exposure to the danger. Young persons are more readily affected than those of mature age, and women more than men. In addition, there seems to be an element of personal susceptibility, the nature of which is not understood. Some persons “work in the lead” for twenty, forty or fifty years without the slightest ill effects; others have attacks whenever they are brought into contact with it. Possibly the difference is due to the general state of health; robust persons resist the poison successfully, those with impoverished blood and feeble constitution are mastered by it. Lead enters the body chiefly through the nose and mouth, being inspired in the form of dust or swallowed with food eaten with unwashed hands. It is very apt to get under the nails, and is possibly absorbed in this way through the skin. Personal care and cleanliness are therefore of the greatest importance. A factory surgeon of great experience in the English Potterieshas stated that seventeen out of twenty cases of lead-poisoning in the china and earthenware industry are due to carelessness (The Times, 8th October 1898).
The Home Office in England has from time to time made special rules for workshops and workpeople, with the object of minimizing or preventing the occurrence of lead-poisoning; and in 1895 notification of cases was made compulsory. The health of workpeople in the Potteries was the subject of a special inquiry by a scientific committee in 1893. The committee stated that “the general truth that the potteries occupation is one fraught with injury to health and life is beyond dispute,” and that “the ill effects of the trade are referable to two chief causes—namely, dust and the poison of lead.” Of these the inhalation of clay and flint dust was the more important. It led to bronchitis, pulmonary tuberculosis and pneumonia, which were the most prevalent disorders among potters, and responsible for 70% of the mortality. That from lead the committee did not attempt to estimate, but they found that plumbism was less prevalent than in past times, and expressed the opinion “that a large part of the mortality from lead poisoning is avoidable; although it must always be borne in mind that no arrangements or rules, with regard to the work itself, can entirely obviate the effects of the poison to which workers are exposed, because so much depends upon the individual and the observance of personal care and cleanliness.” They recommended the adoption of certain special rules in the workshops, with the objects of protecting young persons from the lead, of minimizing the evils of dust, and of promoting cleanliness, particularly in regard to meals. Some of these recommendations were adopted and applied with good results. With regard to the suggestion that “only leadless glazes should be used on earthenware,” they did not “see any immediate prospect of such glazes becoming universally applicable to pottery manufacture,” and therefore turned their attention to the question of “fritting” the lead.
It may be explained that lead is used in china and earthenware to give the external glaze which renders the naturally porous ware watertight. Both “white” and “red” lead are used. The lead is added to other ingredients, which have been “fritted” or fused together and then ground very fine in water, making a thick creamy liquid into which the articles are dipped. After dipping the glaze dries quickly, and on being “fired” in the kiln it becomes fused by the heat into the familiar glassy surface. In the manufacture of ware with enamelled colours, glaze is mixed with the pigment to form a flux, and such colours are used either moist or in the form of a dry powder. “Fritting” the lead means mixing it with the other ingredients of the glaze beforehand and fusing them all together under great heat into a kind of rough glass, which is then ground to make the glaze. Treated in this way the lead combines with the other ingredients and becomes less soluble, and therefore less dangerous, than when added afterwards in the raw state. The committee (1893) thought it “reasonable to suppose that the fritting of lead might ultimately be found universally practicable,” but declared that though fritting “no doubt diminishes the danger of lead-poisoning,” they “could not regard all fritts as equally innocuous.”
It may be explained that lead is used in china and earthenware to give the external glaze which renders the naturally porous ware watertight. Both “white” and “red” lead are used. The lead is added to other ingredients, which have been “fritted” or fused together and then ground very fine in water, making a thick creamy liquid into which the articles are dipped. After dipping the glaze dries quickly, and on being “fired” in the kiln it becomes fused by the heat into the familiar glassy surface. In the manufacture of ware with enamelled colours, glaze is mixed with the pigment to form a flux, and such colours are used either moist or in the form of a dry powder. “Fritting” the lead means mixing it with the other ingredients of the glaze beforehand and fusing them all together under great heat into a kind of rough glass, which is then ground to make the glaze. Treated in this way the lead combines with the other ingredients and becomes less soluble, and therefore less dangerous, than when added afterwards in the raw state. The committee (1893) thought it “reasonable to suppose that the fritting of lead might ultimately be found universally practicable,” but declared that though fritting “no doubt diminishes the danger of lead-poisoning,” they “could not regard all fritts as equally innocuous.”
In the annual report of the chief inspector of factories for 1897, it was stated that there had been “material improvement in dust conditions” in the potting industry, but “of lead-poisoning unfortunately the same could not be said, the number of grave cases reported, and particularly cases of blindness, having ominously increased of late.” This appears to have been largely due to the erroneous inclusion among potting processes of “litho-transfer making,” a colour industry in which girls are employed. New special rules were imposed in 1899 prohibiting the employment of persons under fifteen in the dangerous processes, ordering a monthly examination of all women and young persons working in lead by the certifying surgeon, with power to suspend those showing symptoms of poisoning, and providing for the more effectual removal of dust and the better enforcement of cleanliness. At the same time a scientific inquiry was ordered into the practicability of dispensing with lead in glazes or of substituting fritted compounds for the raw carbonate. The scientific experts reported in 1899, recommending that the use of raw lead should be absolutely prohibited, and expressing the opinion that the greater amount of earthenware could be successfully glazed without any lead. These views were in advance of the opinions held by practical potters, and met with a good deal of opposition. By certain manufacturers considerable progress had been made in diminishing the use of raw lead and towards the discovery of satisfactory leadless glazes; but it is a long step from individual experiments to the wholesale compulsory revolution of the processes of manufacture in so large and varied an industry, and in the face of foreign competitors hampered by no such regulations. The materials used by each manufacturer have been arrived at by a long process of experience, and they are such as to suit the particular goods he supplies for his particular market. It is therefore difficult to apply a uniform rule without jeopardizing the prosperity of the industry, which supports a population of 250,000 in the Potteries alone. However, the bulk of the manufacturers agreed to give up the use of raw lead, and to fritt all their glazes in future, time being allowed to effect the change of process; but they declined to be bound to any particular composition of glaze for the reasons indicated.
In 1901 the Home Office brought forward a new set of special rules. Most of these were framed to strengthen the provisions for securing cleanliness, removing dust, &c., and were accepted with a few modifications. But the question of making even more stringent regulations, even to the extent of making the use of lead-glaze illegal altogether, was still agitated; and in 1906 the Home Office again appointed an expert committee to reinvestigate the subject. They reported in 1910, and made various recommendations in detail for strengthening the existing regulations; but while encouraging the use of leadless glaze in certain sorts of common ceramic ware, they pointed out that, without the use of lead, certain other sorts could either not be made at all or only at a cost or sacrifice of quality which would entail the loss of important markets.
In 1908 Dr Collis made an inquiry into the increase of plumbism in connexion with the smelting of metals, and he considered the increase in the cases of poisoning reported to be due to the third schedule of the Workmen’s Compensation Act, (1) by causing the prevalence of pre-existing plumbism to come to light, (2) by the tendency this fostered to replace men suspected of lead impregnation by new hands amongst whom the incidence is necessarily greater.
In 1908 Dr Collis made an inquiry into the increase of plumbism in connexion with the smelting of metals, and he considered the increase in the cases of poisoning reported to be due to the third schedule of the Workmen’s Compensation Act, (1) by causing the prevalence of pre-existing plumbism to come to light, (2) by the tendency this fostered to replace men suspected of lead impregnation by new hands amongst whom the incidence is necessarily greater.
LEADVILLE,a city and the county seat of Lake county, Colorado, U.S.A., one of the highest (mean elevationc.10,150 ft.) and most celebrated mining “camps” of the world. Pop. (1900) 12,455, of whom 3802 were foreign-born; (1910 census) 7508. It is served by the Denver & Rio Grande, the Colorado & Southern and the Colorado Midland railways. It lies amid towering mountains on a terrace of the western flank of the Mosquito Range at the head of the valley of the Arkansas river, where the river cuts the valley between the Mosquito and the Sawatch (Saguache) ranges. Among the peaks in the immediate environs are Mt. Massive (14,424 ft., the highest in the state) and Elbert Peak (14,421 ft.). There is a United States fish hatchery at the foot of Mt. Massive. In the spring of 1860 placer gold was discovered in California Gulch, and by July 1860 Oro City had probably 10,000 inhabitants. In five years the total yield was more than $5,000,000; then it diminished, and Oro City shrank to a few hundred inhabitants. This settlement was within the present limits of Leadville. In 1876 the output of the mines was about $20,000. During sixteen years “heavy sands” and great boulders that obstructed the placer fields had been moved thoughtlessly to one side. These boulders were from enormous lead carbonate deposits extremely rich in silver. The discovery of these deposits was made on the hills at the edge of Leadville. The first building was erected in June 1877; in December there were several hundred miners, in January the town was organized and named; at the end of 1879 there were, it is said, 35,000 inhabitants. Leadville was already a chartered city, with the usual organization and all public facilities. In 1880 it was reached by the Denver & Rio Grande railway. In early years Leadville was one of the most turbulent, picturesque and in all ways extraordinary, of the mining camps of the West. The value of the output from 1879 to 1889 totalled $147,834,186, including one-fifth of the silver production and a third of the lead consumption of the country. The decline in the price of silver, culminating with the closing of the India mintsand the repeal of the Sherman Law in 1893, threatened Leadville’s future. But the source of the gold of the old placers was found in 1892. From that year to 1899 the gold product rose from $262,692 to $2,183,332. From 1879 to 1900 the camp yielded $250,000,000 (as compared with $48,000,000 of gold and silver in five years from the Comstock, Nevada, lode; and $60,000,000 and 225,000 tons of lead, in fourteen years, from the Eureka, Nevada, mines). Before 1898 the production of zinc was unimportant, but in 1906 it was more valuable than that of silver and gold combined. This increased output is a result of the establishment of concentrating mills, in which the zinc content is raised from 18 or 20% in the raw ores to 25 or 45% in the concentrates. In 1904, per ton of Lake county ore, zinc was valued at $6.93, silver at $4.16, lead at $3.85, gold at $1.77 and copper at $.66. The copper mined at Leadville amounted to about one-third the total mined in the state in 1906. Iron and manganese have been produced here, and in 1906 Leadville was the only place in the United States known to have produced bismuth. There were two famous labour strikes in the “diggings” in 1879 and 1896. The latter attracted national attention; it lasted from the 19th of June 1896 to the 9th of March 1897, when the miners, being practically starved out, declared the strike off. There had been a riot on the 21st of September 1896 and militia guarded the mines for months afterwards. In January 1897 the mines on Carbonate Hill were flooded after the removal of their pumps. This strike closed many mines, which were not opened for several years. Leadville stocks are never on the exchange, and “flotation” and “promotion” have been almost unknown.
The ores of the Leadville District occur in a blue limestone formation overlaid by porphyry, and are in the form of heavy sulphides, containing copper, gold, silver, lead and zinc; oxides containing iron, manganese and small amounts of silver and lead; and siliceous ores, containing much silver and a little lead and gold. The best grade of ores usually consists of a mixture of sulphides, with some native gold. Nowhere have more wonderful advances in mining been apparent—in the size and character of furnaces and pumps; the development of local smelter supplies; the fall in the cost of coal, of explosives and other mine supplies; the development of railways and diminution of freight expenses; and the general improvement of economic and scientific methods—than at Leadville since 1880. The increase of output more than doubled from 1890 to 1900, and many ores once far too low in grade for working now yield sure profits. The Leadville smelters in 1900 had a capacity of 35,000 tons monthly; about as much more local ore being treated at Denver, Pueblo and other places.See S. F. Emmons,Geology and Mining Industry of Leadville, Colorado, monograph United States Geological Survey, vol. 12 (1886), and with J. D. Irving,The Downtown District of Leadville, Colorado, Bulletin 320, United States Geological Survey (1907), particularly for the discussion of the origin of the ores of the region.
The ores of the Leadville District occur in a blue limestone formation overlaid by porphyry, and are in the form of heavy sulphides, containing copper, gold, silver, lead and zinc; oxides containing iron, manganese and small amounts of silver and lead; and siliceous ores, containing much silver and a little lead and gold. The best grade of ores usually consists of a mixture of sulphides, with some native gold. Nowhere have more wonderful advances in mining been apparent—in the size and character of furnaces and pumps; the development of local smelter supplies; the fall in the cost of coal, of explosives and other mine supplies; the development of railways and diminution of freight expenses; and the general improvement of economic and scientific methods—than at Leadville since 1880. The increase of output more than doubled from 1890 to 1900, and many ores once far too low in grade for working now yield sure profits. The Leadville smelters in 1900 had a capacity of 35,000 tons monthly; about as much more local ore being treated at Denver, Pueblo and other places.
See S. F. Emmons,Geology and Mining Industry of Leadville, Colorado, monograph United States Geological Survey, vol. 12 (1886), and with J. D. Irving,The Downtown District of Leadville, Colorado, Bulletin 320, United States Geological Survey (1907), particularly for the discussion of the origin of the ores of the region.
LEAF(O. Eng.léaf, cf. Dutchloof, Ger.Laub, Swed.löf, &c.; possibly to be referred to the root seen in Gr.λέπειν, to peel, strip), the name given in popular language to all the green expanded organs borne upon an axis, and so applied to similar objects, such as a thin sheet of metal, a hinged flap of a table, the page of a book, &c. Investigation has shown that many other parts of a plant which externally appear very different from ordinary leaves are, in their essential particulars, very similar to them, and are in fact their morphological equivalents. Such are the scales of a bulb, and the various parts of the flower, and assuming that the structure ordinarily termed a leaf is the typical form, these other structures were designated changed or metamorphosed leaves, a somewhat misleading interpretation. All structures morphologically equivalent with the leaf are now included under the general termphyllome(leaf-structure).
es, Upper epidermis.
ei, Lower epidermis.
p, Hairs.
st, Stomata.
ps, Upper (palisade) layers of parenchymatous cells.
pi, Lower (spongy) layers of parenchymatous cells.
m, Air-spaces connected with stomata.
l, Air-spaces between the loose cells in the spongy parenchyma.
fv, Bundles of fibro-vascular tissue.
Leaves are produced as lateral outgrowths of the stem in definite succession below the apex. This character, common to all leaves, distinguishes them from other organs. In the higher plants we can easily recognize the distinction between stem and leaf. Amongst the lower plants, however, it is found that a demarcation into stem and leaf is impossible, but that there is a structure which partakes of the characters of both—such is athallus. The leaves always arise from the outer portion of the primary meristem of the plant, and the tissues of the leaf are continuous with those of the stem. Every leaf originates as a simple cellular papilla (fig. 1), which consists of a development from the cortical layers covered by epidermis; and as growth proceeds, the fibro-vascular bundles of the stem are continued outwards, and finally expand and terminate in the leaf. The increase in length of the leaf by growth at the apex is usually of a limited nature. In some ferns, however, there seems to be a provision for indefinite terminal growth, while in others this growth is periodically interrupted. It not unfrequently happens, especially amongst Monocotyledons, that after growth at the apex has ceased, it is continued at the base of the leaf, and in this way the length may be much increased. Amongst Dicotyledons this is very rare. In all cases the dimensions of the leaf are enlarged by interstitial growth of its parts.
The simplest leaf is found in some mosses, where it consists of a single layer of cells. The typicalStructure of leaves.foliage leaf consists of several layers, and amongst vascular plants is distinguishable into an outer layer (epidermis) and a central tissue (parenchyma) with fibro-vascular bundles distributed through it.
Theepidermis(fig. 2,es,ei), composed of cells more or less compressed, has usually a different structure and aspect on the two surfaces of the leaf. The cells of the epidermis are very closely united laterally and contain no green colouring matter (chlorophyll) except in the pair of cells—guard-cells—which bound the stomata. The outer wall, especially of the upper epidermis, has a tough outer layer or cuticle which renders it impervious to water. The epidermis is continuous except where stomata or spaces bounded by specialized cells communicate with intercellular spaces in the interior of the leaf. It is chiefly on the epidermis of the lower surface (fig. 2,ei) that stomata,st, are produced, and it is there also that hairs,p, usually occur. The lower epidermis is often of a dull or pale-green colour, soft and easily detached. The upper epidermis is frequently smooth and shining, and sometimes becomes very hard and dense. Many tropical plants present on the upper surface of their leaves several layers of compressed cells beneath the epidermis which serve for storage of water and are known as aqueous tissue. In leaves which float upon the surface of the water, as those of the water-lily, the upper epidermis alone possesses stomata.Theparenchymaof the leaf is the cellular tissue enclosed within the epidermis and surrounding the vessels (fig. 2,ps,pi). It is known asmesophyll, and is formed of two distinct series of cells, each containing the green chlorophyll-granules, but differing in form and arrangement. Below the epidermis of the upper side of the leaf there are one or two layers of cells, elongated at right angles to the leaf surface (fig. 2,ps), and applied so closely to each other as to leaveonly small intercellular spaces, except where stomata happen to be present (fig. 2,m); they form the palisade tissue. On the other side of the leaf the cells are irregular, often branched, and are arranged more or less horizontally (fig. 2,pi), leaving air-spaces between them,l, which communicate with stomata; on this account the tissue has received the name of spongy. In leaves having a very firm texture, as those of Coniferae and Cycadaceae, the cells of the parenchyma immediately beneath the epidermis are very much thickened and elongated in a direction parallel to the surface of the leaf, so as to be fibre-like. These constitute a hypodermal layer, beneath which the chlorophyll cells of the parenchyma are densely packed together, and are elongated in a direction vertical to the surface of the leaf, forming the palisade tissue. The form and arrangement of the cells, however, depend much on the nature of the plant, and its exposure to light and air. Sometimes the arrangement of the cells on both sides of the leaf is similar, as occurs in leaves which have their edges presented to the sky. In very succulent plants the cells form a compact mass, and those in the centre are often colourless. In some cases the cellular tissue is deficient at certain points, giving rise to distinct holes in the leaf, as inMonstera Adansonii. The fibro-vascular system in the leaf constitutes thevenation. The fibro-vascular bundles from the stem bend out into the leaf, and are there arranged in a definite manner. Inskeleton leaves, or leaves in which the parenchyma is removed, this arrangement is well seen. In some leaves, as in the barberry, the veins are hardened, producing spines without any parenchyma. The hardening of the extremities of the fibro-vascular tissue is the cause of the spiny margin of many leaves, such as the holly, of the sharp-pointed leaves of madder, and of mucronate leaves, or those having a blunt end with a hard projection in the centre.
Theepidermis(fig. 2,es,ei), composed of cells more or less compressed, has usually a different structure and aspect on the two surfaces of the leaf. The cells of the epidermis are very closely united laterally and contain no green colouring matter (chlorophyll) except in the pair of cells—guard-cells—which bound the stomata. The outer wall, especially of the upper epidermis, has a tough outer layer or cuticle which renders it impervious to water. The epidermis is continuous except where stomata or spaces bounded by specialized cells communicate with intercellular spaces in the interior of the leaf. It is chiefly on the epidermis of the lower surface (fig. 2,ei) that stomata,st, are produced, and it is there also that hairs,p, usually occur. The lower epidermis is often of a dull or pale-green colour, soft and easily detached. The upper epidermis is frequently smooth and shining, and sometimes becomes very hard and dense. Many tropical plants present on the upper surface of their leaves several layers of compressed cells beneath the epidermis which serve for storage of water and are known as aqueous tissue. In leaves which float upon the surface of the water, as those of the water-lily, the upper epidermis alone possesses stomata.
Theparenchymaof the leaf is the cellular tissue enclosed within the epidermis and surrounding the vessels (fig. 2,ps,pi). It is known asmesophyll, and is formed of two distinct series of cells, each containing the green chlorophyll-granules, but differing in form and arrangement. Below the epidermis of the upper side of the leaf there are one or two layers of cells, elongated at right angles to the leaf surface (fig. 2,ps), and applied so closely to each other as to leaveonly small intercellular spaces, except where stomata happen to be present (fig. 2,m); they form the palisade tissue. On the other side of the leaf the cells are irregular, often branched, and are arranged more or less horizontally (fig. 2,pi), leaving air-spaces between them,l, which communicate with stomata; on this account the tissue has received the name of spongy. In leaves having a very firm texture, as those of Coniferae and Cycadaceae, the cells of the parenchyma immediately beneath the epidermis are very much thickened and elongated in a direction parallel to the surface of the leaf, so as to be fibre-like. These constitute a hypodermal layer, beneath which the chlorophyll cells of the parenchyma are densely packed together, and are elongated in a direction vertical to the surface of the leaf, forming the palisade tissue. The form and arrangement of the cells, however, depend much on the nature of the plant, and its exposure to light and air. Sometimes the arrangement of the cells on both sides of the leaf is similar, as occurs in leaves which have their edges presented to the sky. In very succulent plants the cells form a compact mass, and those in the centre are often colourless. In some cases the cellular tissue is deficient at certain points, giving rise to distinct holes in the leaf, as inMonstera Adansonii. The fibro-vascular system in the leaf constitutes thevenation. The fibro-vascular bundles from the stem bend out into the leaf, and are there arranged in a definite manner. Inskeleton leaves, or leaves in which the parenchyma is removed, this arrangement is well seen. In some leaves, as in the barberry, the veins are hardened, producing spines without any parenchyma. The hardening of the extremities of the fibro-vascular tissue is the cause of the spiny margin of many leaves, such as the holly, of the sharp-pointed leaves of madder, and of mucronate leaves, or those having a blunt end with a hard projection in the centre.
The form and arrangement of the parts of a typical foliage leaf are intimately associated with the part played by the leaf in the life of the plant. The flat surface is spread to allow the maximum amount of sunlight to fall upon it, as it is by the absorption of energy from the sun’s rays by means of the chlorophyll contained in the cells of the leaf that the building up of plant food is rendered possible; this process is known as photo-synthesis; the first stage is the combination of carbon dioxide, absorbed from the air taken in through the stomata into the living cells of the leaf, with water which is brought into the leaf by the wood-vessels. The wood-vessels form part of the fibro-vascular bundles or veins of the leaf and are continuous throughout the leaf-stalk and stem with the root by which water is absorbed from the soil. The palisade layers of the mesophyll contain the larger number of chlorophyll grains (or corpuscles) while the absorption of carbon dioxide is carried on chiefly through the lower epidermis which is generally much richer in stomata. The water taken up by the root from the soil contains nitrogenous and mineral salts which combine with the first product of photo-synthesis—a carbohydrate—to form more complicated nitrogen-containing food substances of a proteid nature; these are then distributed by other elements of the vascular bundles (thephloem) through the leaf to the stem and so throughout the plant to wherever growth or development is going on. A large proportion of the water which ascends to the leaf acts merely as a carrier for the other raw food materials and is got rid of from the leaf in the form of water vapour through the stomata—this process is known astranspiration. Hence the extended surface of the leaf exposing a large area to light and air is eminently adapted for the carrying out of the process of photo-synthesis and transpiration. The arrangement of the leaves on the stem and branches (seePhyllotaxy, below) is such as to prevent the upper leaves shading the lower, and the shape of the leaf serves towards the same end—the disposition of leaves on a branch or stem is often seen to form a “mosaic,” each leaf fitting into the space between neighbouring leaves and the branch on which they are borne without overlapping.
Submerged leaves, or leaves which are developed under water, differ in structure from aerial leaves. They have usually no fibro-vascular system, but consist of a congeries of cells, which sometimes become elongated and compressed so as to resemble veins. They have a layer of compact cells on their surface, but no true epidermis, and no stomata. Their internal structure consists of cells, disposed irregularly, and sometimes leaving spaces which are filled with air for the purpose of floating the leaf. When exposed to the air these leaves easily part with their moisture, and become shrivelled and dry. In some cases there is only a network of filament-like cells, the spaces between which are not filled with parenchyma, giving a skeleton appearance to the leaf, as inOuvirandra fenestralis(Lattice plant).
A leaf, whether aerial or submerged, generally consists of a flat expanded portion, called theblade, orlamina, of a narrower portion called thepetioleorstalk, and sometimes of a portion at the base of the petiole, which forms asheathorvagina(fig. 5,s), or is developed in the form of outgrowths, calledstipules(fig. 24,s). All these portions are not always present. The sheathing or stipulary portion is frequently wanting. When a leaf has a distinct stalk it ispetiolate; when it has none, it issessile, and if in this case it embraces the stem it is said to beamplexicaul. The part of the leaf next the petiole or the axis is thebase, while the opposite extremity is theapex. The leaf is usually flattened and expanded horizontally,i.e.at right angles to the longitudinal axis of the shoot, so that the upper face is directed towards the heavens, and the lower towards the earth. In some cases leaves, as in Iris, or leaf-like petioles, as in Australian acacias and eucalypti, have their plane of expansion parallel to the axis of the shoot, there is then no distinction into an upper and a lower face, but the two sides are developed alike; or the leaf may have a cylindrical or polyhedral form, as in mesembryanthemum. The upper angle formed between the leaf and the stem is called itsaxil; it is there that leaf-buds are normally developed. The leaf is sometimes articulated with the stem, and when it falls off ascarremains; at other times it is continuous with it, and then decays, while still attached to the axis. In their early state all leaves are continuous with the stem, and it is only in their after growth that articulations are formed. When leaves fall off annually they are calleddeciduous; when they remain for two or more years they arepersistent, and the plant isevergreen. The laminar portion of a leaf is occasionally articulated with the petiole, as in the orange, and a joint at times exists between the vaginal or stipulary portion and the petiole.
The arrangement of the fibro-vascular system in the lamina constitutes thevenationornervation. In an ordinary leaf, as that of the elm, there is observed a large central vein running from the base to the apex of the leaf, this is themidribVenation.(fig. 3); it gives off veins laterally (primary veins). A leaf with only a single midrib is said to beunicostateand the venation is described as pinnate or feather-veined. In some cases, as sycamore or castor oil (fig. 4), in place of there being only a single midrib there are several large veins (ribs) of nearly equal size, which diverge from the point where the blade joins the petiole or stem, giving off lateral veins. The leaf in this case ismulticostateand the venation palmate. The primary veins give off secondary veins, and these in their turn give off tertiary veins, and so on until a complete network of vessels is produced, and those veins usually project on the under surface of the leaf. To a distribution of veins such as this the name ofreticulatedornettedvenation has been applied. In the leaves of some plants there exists a midrib with large veins running nearly parallel to it from the base to the apex of the lamina, as in grasses (fig. 5); or with veins diverging from the base of the lamina in more or lessparallel lines, as in fan palms (fig. 6), or with veins coming off from it throughout its whole course, and running parallel to each other in a straight or curved direction towards the margin of the leaf, as in plantain and banana. In these cases the veins are often united by cross veinlets, which do not, however, form an angular network. Such leaves are said to beparallel-veined. The leaves of Monocotyledons have generally this kind of venation, while reticulated venation most usually occurs amongst Dicotyledons. Some plants, which in most points of their structure are monocotyledonous, yet have reticulated venation; as inSmilaxandDioscorea. In vascular acotyledonous plants there is frequently a tendency to fork exhibited by the fibro-vascular bundles in the leaf; and when this is the case we havefork-veinedleaves. This is well seen in many ferns. The distribution of the system of vessels in the leaf is usually easily traced, but in the case of succulent plants, asHoya, agave, stonecrop and mesembryanthemum, the veins are obscure. The function of the veins which consist of vessels and fibres is to form a rigid framework for the leaf and to conduct liquids.
The arrangement of the fibro-vascular system in the lamina constitutes thevenationornervation. In an ordinary leaf, as that of the elm, there is observed a large central vein running from the base to the apex of the leaf, this is themidribVenation.(fig. 3); it gives off veins laterally (primary veins). A leaf with only a single midrib is said to beunicostateand the venation is described as pinnate or feather-veined. In some cases, as sycamore or castor oil (fig. 4), in place of there being only a single midrib there are several large veins (ribs) of nearly equal size, which diverge from the point where the blade joins the petiole or stem, giving off lateral veins. The leaf in this case ismulticostateand the venation palmate. The primary veins give off secondary veins, and these in their turn give off tertiary veins, and so on until a complete network of vessels is produced, and those veins usually project on the under surface of the leaf. To a distribution of veins such as this the name ofreticulatedornettedvenation has been applied. In the leaves of some plants there exists a midrib with large veins running nearly parallel to it from the base to the apex of the lamina, as in grasses (fig. 5); or with veins diverging from the base of the lamina in more or lessparallel lines, as in fan palms (fig. 6), or with veins coming off from it throughout its whole course, and running parallel to each other in a straight or curved direction towards the margin of the leaf, as in plantain and banana. In these cases the veins are often united by cross veinlets, which do not, however, form an angular network. Such leaves are said to beparallel-veined. The leaves of Monocotyledons have generally this kind of venation, while reticulated venation most usually occurs amongst Dicotyledons. Some plants, which in most points of their structure are monocotyledonous, yet have reticulated venation; as inSmilaxandDioscorea. In vascular acotyledonous plants there is frequently a tendency to fork exhibited by the fibro-vascular bundles in the leaf; and when this is the case we havefork-veinedleaves. This is well seen in many ferns. The distribution of the system of vessels in the leaf is usually easily traced, but in the case of succulent plants, asHoya, agave, stonecrop and mesembryanthemum, the veins are obscure. The function of the veins which consist of vessels and fibres is to form a rigid framework for the leaf and to conduct liquids.
In all plants, except Thallophytes, leaves are present at some period of their existence. InCuscuta(Dodder) (q.v.), however, we have an exception. The forms assumed by leaves vary much, not only in different plants, but in the same plant. It is only amongst the lower classes of plants—Mosses, Characeae, &c.—that all the leaves on a plant are similar. As we pass up the scale of plant life we find them becoming more and more variable. The structures in ordinary language designated as leaves are considered sopar excellence, and they are frequently spoken of asfoliage leaves. In relation to their production on the stem we may observe that when they are small they are always produced in great number, and as they increase in size their number diminishes correspondingly. The cellular process from the axis which develops into a leaf is simple and undivided; it rarely remains so, but in progress of growth becomes segmented in various ways, either longitudinally or laterally, or in both ways. By longitudinal segmentation we have a leaf formed consisting of sheath, stalk and blade; or one or other of these may be absent, and thus stalked, sessile, sheathing, &c., leaves are produced. Lateral segmentation affects the lamina, producing indentations, lobings or fissuring of its margins. In this way two marked forms of leaf are produced—(1)Simpleform, in which the segmentation, however deeply it extends into the lamina, does not separate portions of the lamina which become articulated with the midrib or petiole; and (2)Compoundform, where portions of the lamina are separated as detachedleaflets, which become articulated with the midrib or petiole. In both simple and compound leaves, according to the amount of segmentation and the mode of development of the parenchyma and direction of the fibro-vascular bundles, many forms are produced.
Simple Leaves.—When the parenchyma is developed symmetrically on each side of the midrib or stalk, the leaf isequal; if otherwise, the leaf isunequaloroblique(fig. 3). If the margins are even and present no divisions, the leaf isentire(fig. 7);Simple leaves.if there are slight projections which are more or less pointed, the leaf isdentateor toothed; when the projections lie regularly over each other, like the teeth of a saw, the leaf isserrate(fig. 3); when they are rounded the leaf iscrenate. If the divisions extend more deeply into the lamina than the margin, the leaf receives different names according to the nature of the segments; thus, when the divisions extend about half-way down (fig. 8), it iscleft; when the divisions extend nearly to the base or to the midrib the leaf ispartite.If these divisions take place in a simplefeather-veinedleaf it becomes eitherpinnatifid(fig. 9), when the segments extend to about the middle, orpinnatipartite, when the divisions extend nearly to the midrib. These primary divisions may be again subdivided in a similar manner, and thus a feather-veined leaf will becomebipinnatifidorbipinnatipartite; still further subdivisions give origin totripinnatifidandlaciniatedleaves. The same kinds of divisions taking place in a simple leaf with palmate orradiatingvenation, give origin tolobed,cleftandpartiteforms. The namepalmateorpalmatifid(fig. 4) is the general term applied to leaves with radiating venation, in which there are several lobes united by a broad expansion of parenchyma, like the palm of the hand, as in the sycamore, castor-oil plant, &c. The divisions of leaves with radiating venation may extend to near the base of the leaf, and the namesbipartite,tripartite,quinquepartite, &c., are given according as the partitions are two, three, five or more. The termdissectedis applied to leaves with radiating venation, having numerous narrow divisions, as inGeranium dissectum.Fig.7.Fig.8.Fig.9.Fig.7.—Ovate acute leaf ofCoriara myrtifolia. Besides the midrib there are two intra-marginal ribs which converge to the apex. The leaf is therefore tricostate.Fig.8.—Runcinate leaf of Dandelion. It is a pinnatifid leaf, with the divisions pointing towards the petiole and a large triangular apex.Fig.9.—Pinnatifid leaf ofValeriana dioica.Fig.10.—Five-partite leaf of Aconite.Fig.11.—Pedate leaf of Stinking Hellebore (Helleborus foetidus). The venation is radiating. It is a palmately-partite leaf, in which the lateral lobes are deeply divided. When the leaf hangs down it resembles the foot of a bird, and hence the name.When in a radiating leaf there are three primary partitions, and the two lateral lobes are again cleft, as in hellebore (fig. 11), the leaf is calledpedateorpedatifid, from a fancied resemblance to the claw of a bird. In all the instances already alluded to the leaves have been considered as flat expansions, in which the ribs or veins spread out on the same plane with the stalk. In some cases, however, the veins spread at right angles to the stalk, forming apeltateleaf as in Indian cress (fig. 12).The form of the leaf shows a very great variety ranging from the narrowlinearform with parallel sides, as in grasses or the needle-like leaves of pines and firs to more or less rounded ororbicular—descriptions of these will be found in works on descriptive botany—a fewexamples are illustrated here (figs. 7, 13, 14, 15). The apex also varies considerably, being rounded, orobtuse, sharp oracute(fig. 7), notched (fig. 15), &c. Similarly the shape of the base may vary, when rounded lobes are formed, as in dog-violet, the leaf is cordate or heart-shaped; or kidney-shaped orreniform(fig. 16), when the apex is rounded as in ground ivy. When the lobes are prolonged downwards and are acute, the leaf issagittate(fig. 17); when they proceed at right angles, as inRumex Acetosella, the leaf ishastateor halbert-shaped. When a simple leaf is divided at the base into two leaf-like appendages, it is calledauriculate. When the development of parenchyma is such that it more than fills up the spaces between the veins, the margins becomewavy,crisporundulated, as inRumex crispusandRheum undulatum. By cultivation the cellular tissue is often much increased, giving rise to thecurledleaves of greens, savoys, cresses, lettuce, &c.Fig. 12.—Peltate leaves of Indian Cress (Tropaeolum majus).Fig. 13.—Lanceolate leaf of a species of Senna.Compound leaves are those in which the divisions extend to the midrib or petiole, and the separated portions become each articulated with it, and receive the name ofleaflets. The midrib, or petiole, has thus the appearance of a branch with separate leaves attached to it, but it is considered properly as oneCompound leaves.leaf, because in its earliest state it arises from the axis as a single piece, and its subsequent divisions in the form of leaflets are all in one plane. The leaflets are either sessile (fig. 18) or have stalks, calledpetiolules(fig. 19). Compound leaves are pinnate (fig. 19) or palmate (fig. 18) according to the arrangement of leaflets. When a pinnate leaf ends in a pair of pinnae it isequallyorabruptly pinnate(paripinnate); when there is a single terminal leaflet (fig. 19), the leaf isunequally pinnate(imparipinnate); when the leaflets or pinnae are placed alternately on either side of the midrib, and not directly opposite to each other, the leaf isalternately pinnate; and when the pinnae are of different sizes, the leaf isinterruptedly pinnate. When the division is carried into the second degree, and the pinnae of a compound leaf are themselves pinnately compound, a bipinnate leaf is formed.Fig. 14.Fig. 15.Fig. 16.Fig. 17.Fig. 14.—Oblong leaf of a species of Senna.Fig. 15.—Emarginate leaf of a species of Senna. The leaf in its contour is somewhat obovate, or inversely egg-shaped, and its base is oblique.Fig. 16.—Reniform leaf ofNepeta Glechoma, margin crenate.Fig. 17.—Sagittate leaf of Convolvulus.Fig. 18.—Palmately compound leaf of the Horse-chestnut (Aesculus Hippocastanum).Fig. 19.—Imparipinnate (unequal pinnate) leaf of Robinia. There are nine pairs of shortly-stalked leaflets (foliola, pinnae), and an odd one at the extremity. At the base of the leaf the spiny stipules are seen.Thepetioleor leaf-stalk is the part which unites the limb or blade of the leaf to the stem. It is absent insessileleaves, and this is also frequently the case when a sheath is present, as in grasses (fig. 5). It consists of the fibro-vascular bundles with aPetiole.varying amount of cellular tissue. When the vascular bundles reach the base of the lamina they separate and spread out in various ways, as already described under venation. The lower part of the petiole is often swollen (fig. 20,p), forming thepulvinus, formed of cellular tissue, the cells of which exhibit the phenomenon of irritability. InMimosa pudica(fig. 20) a sensitiveness is located in the pulvinus which upon irritation induces a depression of the whole bipinnate leaf, a similar property exists in the pulvini at the base of the leaflets which fold upwards. The petiole varies in length, being usually shorter than the lamina, but sometimes much longer. In some palms it is 15 or 20 ft. long, and is so firm as to be used for poles or walking-sticks. In general, the petiole is more or less rounded in its form, the upper surface being flattened or grooved. Sometimes it is compressed laterally, as in the aspen, and to this peculiarity the trembling of the leaves of this tree is due. In aquatic plants the leaf-stalk is sometimes distended with air, as inPontederiaandTrapa, so as to float the leaf. At other times it iswinged, and is either leafy, as in the orange (fig. 21,p), lemon andDionaea, or pitcher-like, as inSarracenia(fig. 22). In some Australian acacias, and in some species ofOxalisandBupleurum, the petiole is flattened in a vertical direction, the vascular bundles separating immediately after quitting the stem and running nearly parallel from base to apex. This kind of petiole (fig. 23,p) has been called aphyllode. In these plants the laminae or blades of the leaves are pinnate or bipinnate, and are produced at the extremities of the phyllodes in a horizontal direction; but in many instances they are not developed, and the phyllode serves the purpose of a leaf. These phyllodes, by their vertical position and their peculiar form, give a remarkable aspect to vegetation. On the same acacia there occur leaves with the petiole and lamina perfect; others having the petiole slightly expanded or winged, and the lamina imperfectly developed; and others in which there is no lamina, and the petiole becomes large and broad. Some petioles are long, slender and sensitive to contact, and function as tendrils by means of which the plant climbs; as in the nasturtiums (Tropaeolum), clematis and others; and in compound leaves the midrib and some of the leaflets may similarly be transformed into tendrils, as in the pea and vetch.Fig. 20.—Branch and leaves of the Sensitive plant (Mimosa pudica), showing the petiole in its erect state,a, and in its depressed state,b; also the leaflets closed,c, and the leaflets expanded,d. Irritability resides in the pulvinus,p.The leaf base is often developed as asheath(vagina), which embraces the whole or part of the circumference of the stem (fig. 5). This sheath is comparatively rare in dicotyledons, but is seen in umbelliferous plants. It is much more common amongst monocotyledons. In sedges theLeaf base.sheath forms a complete investment of the stem, whilst in grasses it is split on one side. In the latter plants there is also a membranous outgrowth, theligule, at right angles to the median plane of the leaf from the point where the sheath passes into the lamina, there being no petiole (fig. 5,l).Fig. 21.—Leaf of Orange (Citrus Aurantium), showing a winged leafy petiolep, which is articulated to the laminal.Fig. 22.—Pitcher (ascidium) of a species of Side-saddle plant (Sarracenia purpurea). The pitcher is formed from the petiole, which is prolonged.In leaves in which no sheath is produced we not infrequently find small foliar organs,stipules, at the base of the petiole (fig. 24,s). The stipules are generally two in number, and they are important as supplying characters in certain natural orders. Thus they occurin the pea and bean family, in rosaceous plants and the family Rubiaceae. They are not common in dicotyledons with opposite leaves. Plants having stipules are calledstipulate; those having none areexstipulate. Stipules may be large or small, entire or divided, deciduous or persistent. They are not usually of the same form as the ordinary foliage leaves of the plant, from which they are distinguished by their lateral position at the base of the petiole. In the pansy (fig. 24) the true leaves are stalked and crenate, while the stipulessare large, sessile and pinnatifid. InLathyrus Aphacaand some other plants the true pinnate leaves are abortive, the petiole forms a tendril, and the stipules alone are developed, performing the office of leaves. When stipulate leaves are opposite to each other, at the same height on the stem, it occasionally happens that the stipules on the two sides unite wholly or partially, so as to form aninterpetiolaryorinterfoliarstipule, as in members of the family Rubiaceae. In the case of alternate leaves, the stipules at the base of each leaf are sometimes united to the petiole and to each other, so as to form anadnate,adherentorpetiolarystipule, as in the rose, or anaxillarystipule, as inHouttuynia cordata. In other instances the stipules unite together on the side of the stem opposite the leaf forming anocrea, as in the dock family (fig. 25).Fig. 23.—Leaf of an Acacia (Acacia heterophylla), showing a flattened leaf-like petiolep, called a phyllode, with straight venation, and a bipinnate lamina.In the development of the leaf the stipules frequently play a most important part. They begin to be formed after the origin of the leaves, but grow much more rapidly than the leaves, and in this way they arch over the young leaves and form protective chambers wherein the parts of the leaf may develop. In the figs, magnolia and pondweeds they are very large and completely envelop the young leaf-bud. The stipules are sometimes so minute as to be scarcely distinguishable without the aid of a lens, and so fugacious as to be visible only in the very young state of the leaf. They may assume a hard and spiny character, as inRobinia Pseudacacia(fig. 19), or may be cirrose, as inSmilax, where each stipule is represented by a tendril. At the base of the leaflets of a compound leaf, small stipules (stipels) are occasionally produced.Fig. 24.—Leaf of Pansy.s, Stipules.Fig. 25.—Leaf of Polygonum, with part of stem.o, Ocrea.Variations in the structure and forms of leaves and leafstalks are produced by the increased development of cellular tissue, by the abortion or degeneration of parts, by the multiplication or repetition of parts and by adhesion. When cellular tissue is developed to a great extent, leaves become succulent and occasionallyModifications.assume a crisp or curled appearance. Such changes take place naturally, but they are often increased by the art of the gardener, and the object of many horticultural operations is to increase the bulk and succulence of leaves. It is in this way that cabbages and savoys are rendered more delicate and nutritious. By a deficiency in development of parenchyma and an increase in the mechanical tissue, leaves are liable to become hardened and spinescent. The leaves of barberry and of some species ofAstragalus, and the stipules of the false acacia (Robinia) are spiny. To the same cause is due the spiny margin of the holly-leaf. When two lobes at the base of a leaf are prolonged beyond the stem and unite (fig. 26), the leaf isperfoliate, the stem appearing to pass through it, as inBupleurum perfoliatumandChlora perfoliata; when two leaves unite by their bases they becomeconnate(fig. 27), as inLonicera Caprifolium; and when leaves adhere to the stem, forming a sort of winged or leafy appendage, they aredecurrent, as in thistles. The formation of peltate leaves has been traced to the union of the lobes of a cleft leaf. In the leaf of theVictoria regiathe transformation may be traced during germination. The first leaves produced by the young plant are linear, the second are sagittate and hastate, the third are rounded-cordate and the next are orbicular. The cleft indicating the union of the lobes remains in the large leaves. The parts of the leaf are frequently transformed intotendrils, with the view of enabling the plants to twine round others for support. In Leguminous plants (the pea tribe) the pinnae are frequently modified to form tendrils, as inLathyrus Aphaca, in which the stipules perform the function of true leaves. InFlagellaria indica,Gloriosa superbaand others, the midrib of the leaf ends in a tendril. InSmilaxthere are two stipulary tendrils.Fig. 26.—Perfoliate leaf of a species of Hare’s-ear (Bupleurum rotundifolium). The two lobes at the base of the leaf are united, so that the stalk appears to come through the leaf.Fig. 27.—Connate leaves of a species of Honeysuckle (Lonicera Caprifolium). Two leaves are united by their bases.Fig. 28.—Pitcher of a species of pitcher-plant (Nepenthes distillatoria).The vascular bundles and cellular tissue are sometimes developed in such a way as to form a circle, with a hollow in the centre, and thus give rise to what are calledfistularor hollow leaves, as in the onion, and toascidiaorpitchers. Pitchers are formed either by petioles or by laminae, and they are composed of one or more leaves. InSarracenia(fig. 22) andHeliamphorathe pitcher is composed of the petiole of the leaf. In the pitcher plant,Nepenthes, the pitcher is a modification of the lamina, the petiole often plays the part of a tendril, while the leaf base is flat and leaf-like (fig. 28).InUtriculariabladder-like sacs are formed by a modification of leaflets on the submerged leaves.In some cases the leaves are reduced to merescales—cataphyllaryleaves; they are produced abundantly upon underground shoots. In parasites (Lathraea,Orobanche) and in plants growing on decaying vegetable matter (saprophytes), in which no chlorophyll is formed, these scales are the only leaves produced. InPinusthe only leaves produced on the main stem and the lateral shoots are scales, the acicular leaves of the tree growing from axillary shoots. InCycaswhorls of scales alternate with large pinnate leaves. In many plants, as already noticed, phyllodia or stipules perform the function of leaves. The production of leaf-buds fromleaves sometimes occurs as inBryophyllum, and many plants of the order Gesneraceae. The leaf of Venus’s fly-trap (Dionaea muscipula) when cut off and placed in damp moss, with a pan of water underneath and a bell-glass for a cover, has produced buds from which young plants were obtained. Some species of saxifrage and of ferns also produce buds on their leaves and fronds. InNymphaea micranthabuds appear at the upper part of the petiole.
Simple Leaves.—When the parenchyma is developed symmetrically on each side of the midrib or stalk, the leaf isequal; if otherwise, the leaf isunequaloroblique(fig. 3). If the margins are even and present no divisions, the leaf isentire(fig. 7);Simple leaves.if there are slight projections which are more or less pointed, the leaf isdentateor toothed; when the projections lie regularly over each other, like the teeth of a saw, the leaf isserrate(fig. 3); when they are rounded the leaf iscrenate. If the divisions extend more deeply into the lamina than the margin, the leaf receives different names according to the nature of the segments; thus, when the divisions extend about half-way down (fig. 8), it iscleft; when the divisions extend nearly to the base or to the midrib the leaf ispartite.
If these divisions take place in a simplefeather-veinedleaf it becomes eitherpinnatifid(fig. 9), when the segments extend to about the middle, orpinnatipartite, when the divisions extend nearly to the midrib. These primary divisions may be again subdivided in a similar manner, and thus a feather-veined leaf will becomebipinnatifidorbipinnatipartite; still further subdivisions give origin totripinnatifidandlaciniatedleaves. The same kinds of divisions taking place in a simple leaf with palmate orradiatingvenation, give origin tolobed,cleftandpartiteforms. The namepalmateorpalmatifid(fig. 4) is the general term applied to leaves with radiating venation, in which there are several lobes united by a broad expansion of parenchyma, like the palm of the hand, as in the sycamore, castor-oil plant, &c. The divisions of leaves with radiating venation may extend to near the base of the leaf, and the namesbipartite,tripartite,quinquepartite, &c., are given according as the partitions are two, three, five or more. The termdissectedis applied to leaves with radiating venation, having numerous narrow divisions, as inGeranium dissectum.
Fig.7.—Ovate acute leaf ofCoriara myrtifolia. Besides the midrib there are two intra-marginal ribs which converge to the apex. The leaf is therefore tricostate.
Fig.8.—Runcinate leaf of Dandelion. It is a pinnatifid leaf, with the divisions pointing towards the petiole and a large triangular apex.
Fig.9.—Pinnatifid leaf ofValeriana dioica.
When in a radiating leaf there are three primary partitions, and the two lateral lobes are again cleft, as in hellebore (fig. 11), the leaf is calledpedateorpedatifid, from a fancied resemblance to the claw of a bird. In all the instances already alluded to the leaves have been considered as flat expansions, in which the ribs or veins spread out on the same plane with the stalk. In some cases, however, the veins spread at right angles to the stalk, forming apeltateleaf as in Indian cress (fig. 12).
The form of the leaf shows a very great variety ranging from the narrowlinearform with parallel sides, as in grasses or the needle-like leaves of pines and firs to more or less rounded ororbicular—descriptions of these will be found in works on descriptive botany—a fewexamples are illustrated here (figs. 7, 13, 14, 15). The apex also varies considerably, being rounded, orobtuse, sharp oracute(fig. 7), notched (fig. 15), &c. Similarly the shape of the base may vary, when rounded lobes are formed, as in dog-violet, the leaf is cordate or heart-shaped; or kidney-shaped orreniform(fig. 16), when the apex is rounded as in ground ivy. When the lobes are prolonged downwards and are acute, the leaf issagittate(fig. 17); when they proceed at right angles, as inRumex Acetosella, the leaf ishastateor halbert-shaped. When a simple leaf is divided at the base into two leaf-like appendages, it is calledauriculate. When the development of parenchyma is such that it more than fills up the spaces between the veins, the margins becomewavy,crisporundulated, as inRumex crispusandRheum undulatum. By cultivation the cellular tissue is often much increased, giving rise to thecurledleaves of greens, savoys, cresses, lettuce, &c.
Compound leaves are those in which the divisions extend to the midrib or petiole, and the separated portions become each articulated with it, and receive the name ofleaflets. The midrib, or petiole, has thus the appearance of a branch with separate leaves attached to it, but it is considered properly as oneCompound leaves.leaf, because in its earliest state it arises from the axis as a single piece, and its subsequent divisions in the form of leaflets are all in one plane. The leaflets are either sessile (fig. 18) or have stalks, calledpetiolules(fig. 19). Compound leaves are pinnate (fig. 19) or palmate (fig. 18) according to the arrangement of leaflets. When a pinnate leaf ends in a pair of pinnae it isequallyorabruptly pinnate(paripinnate); when there is a single terminal leaflet (fig. 19), the leaf isunequally pinnate(imparipinnate); when the leaflets or pinnae are placed alternately on either side of the midrib, and not directly opposite to each other, the leaf isalternately pinnate; and when the pinnae are of different sizes, the leaf isinterruptedly pinnate. When the division is carried into the second degree, and the pinnae of a compound leaf are themselves pinnately compound, a bipinnate leaf is formed.
Fig. 14.—Oblong leaf of a species of Senna.
Fig. 15.—Emarginate leaf of a species of Senna. The leaf in its contour is somewhat obovate, or inversely egg-shaped, and its base is oblique.
Fig. 16.—Reniform leaf ofNepeta Glechoma, margin crenate.
Fig. 17.—Sagittate leaf of Convolvulus.
Thepetioleor leaf-stalk is the part which unites the limb or blade of the leaf to the stem. It is absent insessileleaves, and this is also frequently the case when a sheath is present, as in grasses (fig. 5). It consists of the fibro-vascular bundles with aPetiole.varying amount of cellular tissue. When the vascular bundles reach the base of the lamina they separate and spread out in various ways, as already described under venation. The lower part of the petiole is often swollen (fig. 20,p), forming thepulvinus, formed of cellular tissue, the cells of which exhibit the phenomenon of irritability. InMimosa pudica(fig. 20) a sensitiveness is located in the pulvinus which upon irritation induces a depression of the whole bipinnate leaf, a similar property exists in the pulvini at the base of the leaflets which fold upwards. The petiole varies in length, being usually shorter than the lamina, but sometimes much longer. In some palms it is 15 or 20 ft. long, and is so firm as to be used for poles or walking-sticks. In general, the petiole is more or less rounded in its form, the upper surface being flattened or grooved. Sometimes it is compressed laterally, as in the aspen, and to this peculiarity the trembling of the leaves of this tree is due. In aquatic plants the leaf-stalk is sometimes distended with air, as inPontederiaandTrapa, so as to float the leaf. At other times it iswinged, and is either leafy, as in the orange (fig. 21,p), lemon andDionaea, or pitcher-like, as inSarracenia(fig. 22). In some Australian acacias, and in some species ofOxalisandBupleurum, the petiole is flattened in a vertical direction, the vascular bundles separating immediately after quitting the stem and running nearly parallel from base to apex. This kind of petiole (fig. 23,p) has been called aphyllode. In these plants the laminae or blades of the leaves are pinnate or bipinnate, and are produced at the extremities of the phyllodes in a horizontal direction; but in many instances they are not developed, and the phyllode serves the purpose of a leaf. These phyllodes, by their vertical position and their peculiar form, give a remarkable aspect to vegetation. On the same acacia there occur leaves with the petiole and lamina perfect; others having the petiole slightly expanded or winged, and the lamina imperfectly developed; and others in which there is no lamina, and the petiole becomes large and broad. Some petioles are long, slender and sensitive to contact, and function as tendrils by means of which the plant climbs; as in the nasturtiums (Tropaeolum), clematis and others; and in compound leaves the midrib and some of the leaflets may similarly be transformed into tendrils, as in the pea and vetch.
The leaf base is often developed as asheath(vagina), which embraces the whole or part of the circumference of the stem (fig. 5). This sheath is comparatively rare in dicotyledons, but is seen in umbelliferous plants. It is much more common amongst monocotyledons. In sedges theLeaf base.sheath forms a complete investment of the stem, whilst in grasses it is split on one side. In the latter plants there is also a membranous outgrowth, theligule, at right angles to the median plane of the leaf from the point where the sheath passes into the lamina, there being no petiole (fig. 5,l).
In leaves in which no sheath is produced we not infrequently find small foliar organs,stipules, at the base of the petiole (fig. 24,s). The stipules are generally two in number, and they are important as supplying characters in certain natural orders. Thus they occurin the pea and bean family, in rosaceous plants and the family Rubiaceae. They are not common in dicotyledons with opposite leaves. Plants having stipules are calledstipulate; those having none areexstipulate. Stipules may be large or small, entire or divided, deciduous or persistent. They are not usually of the same form as the ordinary foliage leaves of the plant, from which they are distinguished by their lateral position at the base of the petiole. In the pansy (fig. 24) the true leaves are stalked and crenate, while the stipulessare large, sessile and pinnatifid. InLathyrus Aphacaand some other plants the true pinnate leaves are abortive, the petiole forms a tendril, and the stipules alone are developed, performing the office of leaves. When stipulate leaves are opposite to each other, at the same height on the stem, it occasionally happens that the stipules on the two sides unite wholly or partially, so as to form aninterpetiolaryorinterfoliarstipule, as in members of the family Rubiaceae. In the case of alternate leaves, the stipules at the base of each leaf are sometimes united to the petiole and to each other, so as to form anadnate,adherentorpetiolarystipule, as in the rose, or anaxillarystipule, as inHouttuynia cordata. In other instances the stipules unite together on the side of the stem opposite the leaf forming anocrea, as in the dock family (fig. 25).
In the development of the leaf the stipules frequently play a most important part. They begin to be formed after the origin of the leaves, but grow much more rapidly than the leaves, and in this way they arch over the young leaves and form protective chambers wherein the parts of the leaf may develop. In the figs, magnolia and pondweeds they are very large and completely envelop the young leaf-bud. The stipules are sometimes so minute as to be scarcely distinguishable without the aid of a lens, and so fugacious as to be visible only in the very young state of the leaf. They may assume a hard and spiny character, as inRobinia Pseudacacia(fig. 19), or may be cirrose, as inSmilax, where each stipule is represented by a tendril. At the base of the leaflets of a compound leaf, small stipules (stipels) are occasionally produced.
Variations in the structure and forms of leaves and leafstalks are produced by the increased development of cellular tissue, by the abortion or degeneration of parts, by the multiplication or repetition of parts and by adhesion. When cellular tissue is developed to a great extent, leaves become succulent and occasionallyModifications.assume a crisp or curled appearance. Such changes take place naturally, but they are often increased by the art of the gardener, and the object of many horticultural operations is to increase the bulk and succulence of leaves. It is in this way that cabbages and savoys are rendered more delicate and nutritious. By a deficiency in development of parenchyma and an increase in the mechanical tissue, leaves are liable to become hardened and spinescent. The leaves of barberry and of some species ofAstragalus, and the stipules of the false acacia (Robinia) are spiny. To the same cause is due the spiny margin of the holly-leaf. When two lobes at the base of a leaf are prolonged beyond the stem and unite (fig. 26), the leaf isperfoliate, the stem appearing to pass through it, as inBupleurum perfoliatumandChlora perfoliata; when two leaves unite by their bases they becomeconnate(fig. 27), as inLonicera Caprifolium; and when leaves adhere to the stem, forming a sort of winged or leafy appendage, they aredecurrent, as in thistles. The formation of peltate leaves has been traced to the union of the lobes of a cleft leaf. In the leaf of theVictoria regiathe transformation may be traced during germination. The first leaves produced by the young plant are linear, the second are sagittate and hastate, the third are rounded-cordate and the next are orbicular. The cleft indicating the union of the lobes remains in the large leaves. The parts of the leaf are frequently transformed intotendrils, with the view of enabling the plants to twine round others for support. In Leguminous plants (the pea tribe) the pinnae are frequently modified to form tendrils, as inLathyrus Aphaca, in which the stipules perform the function of true leaves. InFlagellaria indica,Gloriosa superbaand others, the midrib of the leaf ends in a tendril. InSmilaxthere are two stipulary tendrils.
The vascular bundles and cellular tissue are sometimes developed in such a way as to form a circle, with a hollow in the centre, and thus give rise to what are calledfistularor hollow leaves, as in the onion, and toascidiaorpitchers. Pitchers are formed either by petioles or by laminae, and they are composed of one or more leaves. InSarracenia(fig. 22) andHeliamphorathe pitcher is composed of the petiole of the leaf. In the pitcher plant,Nepenthes, the pitcher is a modification of the lamina, the petiole often plays the part of a tendril, while the leaf base is flat and leaf-like (fig. 28).
InUtriculariabladder-like sacs are formed by a modification of leaflets on the submerged leaves.
In some cases the leaves are reduced to merescales—cataphyllaryleaves; they are produced abundantly upon underground shoots. In parasites (Lathraea,Orobanche) and in plants growing on decaying vegetable matter (saprophytes), in which no chlorophyll is formed, these scales are the only leaves produced. InPinusthe only leaves produced on the main stem and the lateral shoots are scales, the acicular leaves of the tree growing from axillary shoots. InCycaswhorls of scales alternate with large pinnate leaves. In many plants, as already noticed, phyllodia or stipules perform the function of leaves. The production of leaf-buds fromleaves sometimes occurs as inBryophyllum, and many plants of the order Gesneraceae. The leaf of Venus’s fly-trap (Dionaea muscipula) when cut off and placed in damp moss, with a pan of water underneath and a bell-glass for a cover, has produced buds from which young plants were obtained. Some species of saxifrage and of ferns also produce buds on their leaves and fronds. InNymphaea micranthabuds appear at the upper part of the petiole.
Leaves occupy various positions on the stem and branches,Phyllotaxis.and have received different names according to their situation. Thus leaves arising from the crown of the root, as in the primrose, are calledradical; those on the stem arecauline; on flower-stalks,floralleaves (seeFlower). The first leaves developed are known as seed leaves orcotyledons. The arrangement of the leaves on the axis and its appendages is calledphyllotaxis.