The parts of a plant concerned in its nutrition are the root, stem, and leaf, which are termed the organs of vegetation. The leaf, as is the case with the others, is subject to many modifications, and is indeed sometimes made to serve other purposes than those of foliage; but its chief function is that of elaborating the crude material supplied by the roots and absorbed from the air into substances which will nourish and continue the growth of the plant. This work requires the light of the sun, and one of the processes incidental to it is evaporation; hence the leaf is generally constructed and arranged upon the stem in such a manner as to expose the largest surface to the influence of light, and usually presents a broad evaporating and absorbing surface to the atmosphere. In its most complete form the leaf consists of an expanded portion, the blade or limb, which is attached to the stem by means of a leaf stalk or petiole, and at the base of this there are two foliaceous appendages or stipules.
The stipules are characteristic of some families of plants and are always present in them, but in other families they are entirely wanting, and hence cannot be regarded as essential to the leaf; so with the petiole, which is frequently absent, the blade being attached directly to the stem, or sessile. The blade is regarded as the only essential part of the leaf, and though this presents itself in a vast variety of forms, the same general structure is manifest in all. In all ordinary leaves two distinct structures are visible: a framework or skeleton of fibres, and a green pulpy portion which fills the spaces in this. "When a principal bundle of fibres runs from the base to the apex of the leaf, it is termed the midrib; the branches from this are called veins, and the smaller subdivisions veinlets. In many leaves the smaller vein-lets anastomose and thus form a complete network; in others the veins run parallel and do not anastomose; as a general rule netted-veined leaves are found in dicotyledonous, and parallel-veined leaves in monocotyledonous plants.
The form of the leaf largely depends upon the disposition of the veins; if there is a midrib with smaller branches or veins from each side, the leaf is said to be pinnately veined, and is usually longer than broad; but if there are several principal ribs starting from the base of the leaf, it is palmately veined, and its outline will be more or less orbicular. The skeleton or framework of the leaf consists of proper wood, and the microscope shows the various vessels, ducts, and fibres found in the stem itself; and this portion of the leaf is regarded as an expansion of the woody system of the stem, or rather of the inner bark. Its structure is beautifully shown in what are called skeletonized leaves, often seen as parlor ornaments; these are prepared by macerating the leaves in water until the softer parts have decayed, and arresting the process while the fibres still remain intact. The pulpy portion of the leaf, cellular tissue or parenchyma, consists of several layers of cells containing chlorophyl or leaf-green; those nearest the upper surface are elongated and packed close together with but few spaces between them, while the cells of the lower part are irregular in shape and placed loosely together to leave abundant air spaces among them.
The darker green color of the upper surfaces of most leaves is due to the more compact character of the cellular tissue in that part of the leaf. This portion of the leaf is regarded as an expansion of the green layer of the bark. Both surfaces of the leaf are covered by an epidermis consisting of empty, thick-walled cells, which cohere so firmly that it may often be stripped off from the other portions of the leaf; the cells of the epidermis are frequently very irregular in outline, and are mostly in a single layer, but in plants which have to resist long droughts there are several layers. The epidermis being impermeable, there could be no communication between the interior of the leaf and the atmosphere were it not for the multitude of breathing pores or stomata provided for this purpose. Each of these stomata is guarded by a pair of curved cells, which, unlike those of the epidermis, contain chlorophyl; these cells are sensitive to the action of moisture, and by their change in form enlarge or diminish the opening. Through these pores the air has direct access to the spaces among the cells of the leaf, and as these are mostly near the lower surface, so the stomata are most numerous in that portion of the leaf.
The stomata in some plants are 20 times more numerous in the epidermis of the lower than in that of the upper surface. The number is estimated to vary in different plants from 800 to 170,000 to the square inch of surface. The epidermis has its cells often prolonged into hairs of various shapes. By careful manipulation a transverse section of a leaf may be made, and this examined with a microscope of moderate power will show, first, a layer of empty cells of the epidermis; next, elongated cells, containing chlorophyl, with their longer diameters placed transversely and closely compacted; then similar cells with their long diameters parallel to the face of the leaf, with numerous air spaces among them; and finally, the epidermis of the lower surface. - The forms assumed by simple leaves are almost innumerable, and as they are much used in systematic botany in the determination of species, they have a technical nomenclature which it would be out of place to give here. The two principal divisions of pin-nately and palmately veined have been mentioned. Each kind of venation in its modifications gives rise to two sets of forms as to general outline, and we have every gradation from the narrowly linear leaves of the grasses to the orbicular and kidney-shaped leaves.
By modifications of the base of the leaf a set of forms, as arrow-shaped and heart-shaped, are produced, and by changes in the apex another set, from the long acuminate to the abruptly truncate. Changes in the margin are innumerable; in many leaves the edges are entire, but more frequently they are finely or coarsely serrate, toothed, or lobed, and the blade of the leaf is sometimes lobed or divided quite down to the midrib; the pinnately and palmately veined leaves when lobed giving two distinct sets of forms. So both these kinds of leaves are often compound, i. e., made up of smaller leaves or leaflets, which are articulated with a common petiole. Among trees, the locust is an example of the pinnate and the horse chestnut of the palmate compound leaves. Sometimes a stem appears to pass directly through the blade of a leaf; to such the name of perfoliate has been given; in some cases this appearance is produced by the lobes of a sessile heart-shaped leaf uniting to enclose the stem, and in others by the union of the bases of two opposite leaves. - The petiole is essentially of the same structure as the leaf, but its cellular tissue is usually small in proportion to its woody portion; its size and length, in proportion to the leaf, vary greatly; in aquatic plants with floating leaves it is several feet long, and in some palms the petiole is so large as to serve for making oars; in our garden rhubarb it takes on an unusual development, and is the useful part of the plant.
In some plants the petiole is so short as to be barely perceptible, and in many entirely absent. In the aspen the petiole is flattened at right angles to the axis of the leaf, which allows the leaf to move with the slightest motion of the air, and to keep up the continuous fluttering for which the foliage of this tree is proverbial. The petiole may be channelled, or furnished with a wing on each side, as in the orange, or it may be broadly expanded at the base and sheathe the stem, as in the umbelliferce, thus when present affording useful characters in describing plants. In some plants the blade of the leaf is wanting, but the petiole expands and becomes leaf-like, and takes upon itself the functions of the leaf; it is then called a phyllodium. The Australian acacias afford numerous examples of phyl-lodia; several of these, when raised from seed, produce upon the young plants the compound leaves common to the genus; some species, when but a few inches high, show a tendency to suppress the blade of the leaf; the successive leaves have wider and wider petioles and less and less blade, until the leaf-like petioles or phyllodia constitute the sole foliage; these have parallel veins and their edges instead of their surfaces are presented to the sky and earth. - Stipules, the accessory leaf-like bodies found in many leaves at the base of the petiole, present great variety in size, form, and duration; they frequently fall away as the leaves expand; in our tulip tree (liriodendrori), and in the related magnolias, they are only to be found as the leaves are unfolding, and are then very conspicuous; again, they remain as long as the leaf to which they belong, and often form a large part of it, as in the garden pea, and in a related species the blade of the leaf is wanting, and the whole foliage of the plant consists of stipules.
In some cases the stipules are distinct, but in many, as in the rose, they are attached to the petiole by one edge; in the docks, rhubarb, and other members of the polygonum family, they are united by both margins, and thus form a sheath which surrounds the stem. The appearance of stipules in the form of spines is not rare, and is noticeable in the common locust, the caper, and other plants. Those parasitic plants which, like the dodder, rob other plants of elaborated food, have no leaves, their function being performed by the foliage of the host to which they are attached. The prickly pear and others of the cactus family are usually regarded as leafless; but these have minute leaves upon the young stems, which soon drop, and in the older ones the whole surface of the stem performs the functions of the leaf. - Leaves vary greatly in size, but generally what is lacking in size is made up in number; and thus trees with minute leaves, like the arbor vitas, where they are like small green scales clothing the branches, present in the aggregate as large a surface of foliage as trees with much larger leaves.
A little plant of our fresh-water ponds has leaves only 1/36 of an inch long, while those of the Victoria regia, of the South American lakes, have a diameter of 6 ft., and afford a standing place for aquatic birds while they are watching for their prey. Some palms have leaves of enormous size, and an arad discovered a few years ago in Central America (Godwinia gigas) has leaves over 13 ft. long. - Leaves differ greatly as to their duration; some are fugacious, falling soon after they appear; those which fall at the close of the season are deciduous, and when they remain through the year they are persistent, as in the evergreens. In some of the evergreen coniferous trees the leaves of the former year fall as soon as those of the present year are developed, while in some firs they remain 10 or 12 years before they fall. In deciduous trees the fall of the leaf is as well provided for as its development, and is not due, as is supposed by some, to the advent of frost. A distinct line of separation is early visible, and before the leaf separates the wound which would otherwise be left is covered with a prolongation of the epidermis of the stem.
The leaf scars in the horse chestnut and ailantus are large, and show the points from which issued the bundles of woody fibre to form the framework of the leaf. In palms and some other monocotyledonous plants the leaves do not fall, but wither and decay upon the tree. - The study of the morphology of the leaf presents an endless variety to a close observer, and nothing in relation to the subject is more interesting than the many abnormal forms in which leaves, besides performing their proper offices as foliage, are made to serve the plants in other respects, or are quite turned aside from their normal uses to those which ordinarily belong to the root or stem. The scales which make up the greater part of a lily bulb are only the bases of former leaves which have become thick and fleshy by the accumulation of nutriment which is to be used in the future growth of the plant. This conversion of leaves into storehouses of food is strikingly shown in some seeds, in many of which the first leaves of the embryo plant, the cotyledons, or seed leaves as they are popularly called, are quite distorted by the accumulation of starchy and other matters intended to nourish the young plant; the common bean is a familiar illustration of this; in the bean the seed leaves fall away after they have parted with their store, but in the squash and others of its family the seed leaves, after they have served their purpose of helping the growth of the young plant, increase in size, turn green, and become proper leaves.
The seed leaves of the oak, pea, and others are so distorted by the food they contain that they never come to the light and appear as proper leaves. The scales which surround the buds of deciduous trees are only modified leaves, some trees showing a regular gradation from the brown dry scale to the fully developed green leaf. In the barberry the leaves often appear as spines, and in Fouquieria, one of the chaparral plants of western Texas, the stem of which is formidably covered with sharp points, the spine is the midrib of the leaf from which the blade has fallen away. The common pea affords an illustration of the conversion of a portion of the leaf into a tendril to aid the plant in climbing, and in a plant of the same family (lathy-rus aphacd) the whole leaf is developed as a tendril. Among the abnormal forms of leaves, none are more interesting than the ascidia or pitchers, in which, as in our native pitcher plants (Sarracenia), the whole leaf forms a pitcher, or, as in nepenthes, the pitcher is an appendage to the leaf. (See Pitcher Plants.) Still more wonderful is the adaptation of the leaf to serve as a trap to catch insects, as in our carnivorous Venus's fly-trap. (See Dio-nAea.) - In this brief description of the leaf it has been considered in only its normal condition of foliage and some of its readily understood transformations, but the botanist regards the flower and its resulting fruit of whatever kind as only peculiar modifications of the leaf.
The idea of tracing all the floral organs to one type, the leaf, had been hinted at before Linnaeus, and the great botanist himself did not present the matter in such a way as to attract much attention; the poet Goethe proposed the theory in much the same form as it is now held, but it was not until the elder De Can-dolle presented it that the theory of metamorphosis was generally accepted. - The arrangement of the leaves upon the stem is such as to give the greatest possible amount of divergence, and though they may appear to be scattered without order, they are arranged in a manner definite for such species, and this has given rise to a distinct department of botany involving mathematical principles, called phyllotaxy. - It has already been hinted that the chief function of the leaf is to bring the interior of the plant into communication with the sun and air; it receives the liquid taken up by the roots, and in its tissues evaporation goes on ; it not only permits but regulates evaporation by the wonderful mechanism of the leaf pores.
But evaporation is by no means the sole office of the leaf; the air with its gases has free access to its interior, and here takes place the process of assimilation, about which so little is known, in which carbon, oxygen, hydrogen, and nitrogen are converted into organic compounds. We know that the chlorophyl is an active agent in effecting these changes, and that sunlight is essential to the proper performance of the leaf's functions. As water is so largely evaporated in the leaf, and as this, coming from the soil, must contain more or less inorganic matter in solution, it is not surprising to find that the leaf contains a large amount of earthy matter or ash, and that leaves in autumn have a larger percentage of ash than vernal leaves; the leaves show upon analysis 10 to 30 times as much ash as the wood of the same tree.
Fig. 1. - Pinnately veined Leaf, with Petiole and Stipules.
Fig. 2. - Palmately veined Leaf (Maple).
Fig. 3. - Parallel-veined Leaf.
Fig. 4. - Stomata of Epidermis (magnified).
Fig. 5. - Cross Section of Leaf (magnified).
Fig. 6. - Acacia, with Pinnate Leaves and Phyllodia.
Fig. 7. - Votch with Leaf developed as Tendril.
Fig. 8. - Barberry with Spiny Leaves.