Chapter VI: The Language as Recorded (13)

The metal itself is not used in medicine. The chief pharmacopoeial salts are: (1) _Plumbi oxidum_ (lead oxide), litharge. It is not used internally, but from it is made _Emplastrum Plumbi_ (diachylon plaster), which is an oleate of lead and is contained in emplastrum hydrargeri, emplastrum plumbi iodidi, emplastrum resinae, emplastrum saponis. (2) _Plumbi Acetas_ (sugar of lead), dose 1 to 5 grains. From this salt are made the following preparations: (a) _Pilula Plumbi cum Opio_, the strength of the opium in it being 1 in 8, dose 2 to 4 grains; (b) _Suppositoria Plumbi composita_, containing lead acetate, opium and oil of theobroma, there being one grain of opium in each suppository; (c) _Unguentum Plumbi Acetatis_; (d) _Liquor Plumbi Subacetatis Fortior_, Goulard's extract, strength 24% of the subacetate; this again has a sub-preparation, the _Liquor Plumbi Subacetatis Dilutis_, called Goulard's water or Goulard's lotion, containing 1 part in 80 of the strong extract; (e) _Glycerinum Plumbi Subacetatis_, from which is made the _Unguentum Glycerini Plumbi Subacetatis_. (3) _Plumbi Carbonas_, white lead, a mixture of the carbonate and the hydrate, a heavy white powder insoluble in water; it is not used internally, but from it is made _Unguentum Plumbi Carbonatis_, strength 1 in 10 parts of paraffin ointment. (4) _Plumbi Iodidium_, a heavy bright yellow powder not used internally. From it are made (a) _Emplastrum Plumbi Iodidi_, and (b) _Unguentum Plumbi Iodidi_. The strength of each is 1 in 10.

Applied externally lead salts have practically no action upon the unbroken skin, but applied to sores, ulcers or any exposed mucous membranes they coagulate the albumen in the tissues themselves and contract the small vessels. They are very astringent, haemostatic and sedative; the strong solution of the subacetate is powerfully caustic and is rarely used undiluted. Lead salts are applied as lotions in conditions where a sedative astringent effect is desired, as in weeping eczema; in many varieties of chronic ulceration; and as an injection for various inflammatory discharges from the vagina, ear and urethra, the Liquor Plumbi Subacetatis Dilutum being the one employed. The sedative effect of lead lotion in pruritus is well known. Internally lead has an astringent action on the mucous membranes, causing a sensation of dryness; the dilute solution of the subacetate forms an effective gargle in tonsillitis. The chief use of the preparations of lead, however, is as an astringent in acute diarrhoea, particularly if ulceration be present, when it is usefully given in combination with opium in the form of the Pilula Plumbi cum Opio. It is useful in haemorrhage from a gastric ulcer or in haemorrhage from the intestine. Lead salts usually produce constipation, and lead is an active ecbolic. Lead is said to enter the blood as an albuminate in which form it is deposited in the tissues. As a rule the soluble salts if taken in sufficient quantities produce acute poisoning, and the insoluble salts chronic plumbism. The symptoms of acute poisoning are pain and diarrhoea, owing to the setting up of an active gastro-enteritis, the foeces being black (due to the formation of a sulphide of lead), thirst, cramps in the legs and muscular twitchings, with torpor, collapse, convulsions and coma. The treatment is the prompt use of emetics, or the stomach should be washed out, and large doses of sodium or magnesium sulphate given in order to form an insoluble sulphate. Stimulants, warmth and opium may be required. For an account of chronic plumbism see LEAD POISONING.

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.

See _The Life and Work of B. W. Leader, R.A._, by Lewis Lusk, _Art
Journal_ Office (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, or PLUMBISM, 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.

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 Potteries has 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."

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.

LEADVILLE, a city and the county seat of Lake county, Colorado, U.S.A., one of the highest (mean elevation c. 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 mints and 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.

LEAF (O. Eng. _léaf_, cf. Dutch _loof_, Ger. _Laub_, Swed. _löf_, &c.; possibly to be referred to the root seen in Gr. [Greek: lepein], 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 term _phyllome_ (leaf-structure).

FIG. 1.--Apex of a shoot showing origin of leaves: f, leaf rudiment; g, rudiment of an axillary bud.]

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 a _thallus_. 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.

Structure of leaves.

The simplest leaf is found in some mosses, where it consists of a single layer of cells. The typical 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.

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.]

The _epidermis_ (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.

The _parenchyma_ of the leaf is the cellular tissue enclosed within
the epidermis and surrounding the vessels (fig. 2, ps, pi). It is
known as _mesophyll_, 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
leave only 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 in _Monstera Adansonii_. The
fibro-vascular system in the leaf constitutes the _venation_. The
fibro-vascular bundles from the stem bend out into the leaf, and are
there arranged in a definite manner. In _skeleton 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 (the _phloem_) 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 as _transpiration_. 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 (see _Phyllotaxy_, 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 in _Ouvirandra fenestralis_ (Lattice plant).

A leaf, whether aerial or submerged, generally consists of a flat expanded portion, called the _blade_, or _lamina_, of a narrower portion called the _petiole_ or _stalk_, and sometimes of a portion at the base of the petiole, which forms a _sheath_ or _vagina_ (fig. 5, s), or is developed in the form of outgrowths, called _stipules_ (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 is _petiolate_; when it has none, it is _sessile_, and if in this case it embraces the stem it is said to be _amplexicaul_. The part of the leaf next the petiole or the axis is the _base_, while the opposite extremity is the _apex_. 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 its _axil_; it is there that leaf-buds are normally developed. The leaf is sometimes articulated with the stem, and when it falls off a _scar_ remains; 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 called _deciduous_; when they remain for two or more years they are _persistent_, and the plant is _evergreen_. 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.

Venation.

The arrangement of the fibro-vascular system in the lamina constitutes
the _venation_ or _nervation_. 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 the _midrib_ (fig. 3); it gives off
veins laterally (_primary veins_). A leaf with only a single midrib is
said to be _unicostate_ and 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 is _multicostate_ and 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 of
_reticulated_ or _netted_ venation 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 less parallel 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 be _parallel-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 in _Smilax_ and _Dioscorea_. 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 have _fork-veined_ leaves. 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, as _Hoya_, 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. In _Cuscuta_ (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 so _par excellence_, and they are frequently spoken of as _foliage 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) _Simple_ form, 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) _Compound_ form, where portions of the lamina are separated as detached _leaflets_, 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.

_Simple Leaves._--When the parenchyma is developed symmetrically on
each side of the midrib or stalk, the leaf is _equal_; if otherwise,
the leaf is _unequal_ or _oblique_ (fig. 3). If the margins are even
and present no divisions, the leaf is _entire_ (fig. 7); if there are
slight projections which are more or less pointed, the leaf is
_dentate_ or toothed; when the projections lie regularly over each
other, like the teeth of a saw, the leaf is _serrate_ (fig. 3); when
they are rounded the leaf is _crenate_. 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 is _cleft_; when the
divisions extend nearly to the base or to the midrib the leaf is
_partite_.

If these divisions take place in a simple _feather-veined_ leaf it
becomes either _pinnatifid_ (fig. 9), when the segments extend to
about the middle, or _pinnatipartite_, 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 become
_bipinnatifid_ or _bipinnatipartite_; still further subdivisions give
origin to _tripinnatifid_ and _laciniated_ leaves. The same kinds of
divisions taking place in a simple leaf with palmate or _radiating_
venation, give origin to _lobed_, _cleft_ and _partite_ forms. The
name _palmate_ or _palmatifid_ (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
names _bipartite_, _tripartite_, _quinquepartite_, &c., are given
according as the partitions are two, three, five or more. The term
_dissected_ is applied to leaves with radiating venation, having
numerous narrow divisions, as in _Geranium dissectum_.

FIG. 7.--Ovate acute leaf of _Coriara 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 of _Valeriana 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 called _pedate_ or _pedatifid_, 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 a _peltate_
leaf as in Indian cress (fig. 12).

The form of the leaf shows a very great variety ranging from the
narrow _linear_ form with parallel sides, as in grasses or the
needle-like leaves of pines and firs to more or less rounded or
_orbicular_--descriptions of these will be found in works on
descriptive botany--a few examples are illustrated here (figs. 7, 13,
14, 15). The apex also varies considerably, being rounded, or
_obtuse_, sharp or _acute_ (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 or
_reniform_ (fig. 16), when the apex is rounded as in ground ivy. When
the lobes are prolonged downwards and are acute, the leaf is
_sagittate_ (fig. 17); when they proceed at right angles, as in _Rumex
Acetosella_, the leaf is _hastate_ or halbert-shaped. When a simple
leaf is divided at the base into two leaf-like appendages, it is
called _auriculate_. When the development of parenchyma is such that
it more than fills up the spaces between the veins, the margins become
_wavy_, _crisp_ or _undulated_, as in _Rumex crispus_ and _Rheum
undulatum_. By cultivation the cellular tissue is often much
increased, giving rise to the _curled_ leaves of greens, savoys,
cresses, lettuce, &c.

Compound leaves.

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 of _leaflets_. The midrib, or petiole, has
thus the appearance of a branch with separate leaves attached to it,
but it is considered properly as one 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, called _petiolules_ (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 is _equally_ or _abruptly pinnate_ (paripinnate);
when there is a single terminal leaflet (fig. 19), the leaf is
_unequally 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 is _alternately pinnate_; and when
the pinnae are of different sizes, the leaf is _interruptedly
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 of _Nepeta Glechoma_, margin crenate.

FIG. 17.--Sagittate leaf of Convolvulus.]

Petiole.

The _petiole_ or leaf-stalk is the part which unites the limb or blade
of the leaf to the stem. It is absent in _sessile_ leaves, 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 a 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 the _pulvinus_, formed of cellular
tissue, the cells of which exhibit the phenomenon of irritability. In
_Mimosa 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 in _Pontederia_ and
_Trapa_, so as to float the leaf. At other times it is _winged_, and
is either leafy, as in the orange (fig. 21, p), lemon and _Dionaea_,
or pitcher-like, as in _Sarracenia_ (fig. 22). In some Australian
acacias, and in some species of _Oxalis_ and _Bupleurum_, 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 a
_phyllode_. 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.

Leaf base.

The leaf base is often developed as a _sheath_ (_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 the 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, the _ligule_, 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 occur in
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 called _stipulate_; those having none are
_exstipulate_. 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
stipules s are large, sessile and pinnatifid. In _Lathyrus Aphaca_ and
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 an
_interpetiolary_ or _interfoliar_ stipule, 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 an _adnate_, _adherent_ or _petiolary_ stipule, as in the
rose, or an _axillary_ stipule, as in _Houttuynia cordata_. In other
instances the stipules unite together on the side of the stem opposite
the leaf forming an _ocrea_, 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 in _Robinia Pseudacacia_ (fig. 19), or may be cirrose,
as in _Smilax_, where each stipule is represented by a tendril. At the
base of the leaflets of a compound leaf, small stipules (_stipels_)
are occasionally produced.

Modifications.

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 occasionally 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 of
_Astragalus_, 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 is _perfoliate_, the stem appearing to pass
through it, as in _Bupleurum perfoliatum_ and _Chlora perfoliata_;
when two leaves unite by their bases they become _connate_ (fig. 27),
as in _Lonicera Caprifolium_; and when leaves adhere to the stem,
forming a sort of winged or leafy appendage, they are _decurrent_, 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 the _Victoria
regia_ the 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 into
_tendrils_, 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 in _Lathyrus Aphaca_, in
which the stipules perform the function of true leaves. In
_Flagellaria indica_, _Gloriosa superba_ and others, the midrib of the
leaf ends in a tendril. In _Smilax_ there 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 called _fistular_ or hollow leaves, as in the
onion, and to _ascidia_ or _pitchers_. Pitchers are formed either by
petioles or by laminae, and they are composed of one or more leaves.
In _Sarracenia_ (fig. 22) and _Heliamphora_ the 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).

In _Utricularia_ bladder-like sacs are formed by a modification of
leaflets on the submerged leaves.