But the service of leaves in the nutrition of the plant does not cease here. They perform a most important function in the transpiration of water. Plants must always draw from the soil a quantity of water far in excess of their needs, or of their capacity to hold, in order to get a sufficient supply of the mineral food dis solved in it, but in exceedingly small quantities; and after that sustenance has been extracted the extra useless surplus of water must be got rid of. This is accomplished through the stomata of the leaves, out of which water is always passing in gaseous evaporation or some times even in globules. A secondary but most important accompaniment of this is the suction thus formed, by which the constant up-flow from the root-ends is maintained.
A third essential office of leaves is as the lungs of the plant, which must breathe in essen tially the same manner and for the same pur poses as does an animal; that is, they must take up oxygen and give off carbonic acid. This independent process (the converse of the simul taneous assimilation) is carried on steadily by all plants, night and day; but in those having leaves it is mainly performed by these organs, because they spread the greatest surface.
In addition to these foremost and general services, leaves are adapted in particular cases, almost as numerous as the plant species, to such special purposes as a depository of food for the young plant in the cotyledons or seed-leaves; as bulb-scales in plants like the hyacinth and lily, where part of the nourishment in the foliage of one year is stored up in the scales or subterra nean thickened leaves, for the early growth and flowering of the next year; as bud-scales, form ing the protective coverings of buds, as tendrils, pitchers, fly-.traps, etc.
These complicated requirements and duties, under varied conditions and circumstances, have produced the extraordinary modifications of form and texture which leaves present, and which must now be briefly considered.
Forms and Arrangement of typical and ordinary foliage leaf is a thin, flat structure composed of stalk (petiole) and blade (lamina) of symmetrical form, and growing in the plane of the horizon, so that one side (the dorsal) is presented upward to the sky and sun shine, and the other (ventral) is downward and in shadow; and these sides usually present ap propriate differences in texture, the upper sur face being usually more smooth and compact than the lower. A great variety of textures, from smooth, polished or, to rough, downy or spiny, are distinguished by botanists and used in the description of plants; these variations of surface are largely defensive in their character. Some leaves have no stalk and are said to be sessile, in which case the base of the leaf may partly clasp or completely sur round the stem, or be otherwise modified; similarly the stalk takes many forms, sometimes with two lesser subsidiary leaves (stipules) at the base. The rigid woody centre of the stalk may continue straight on through the middle of the leaf to its apex, forming a midrib which throws out branches alternately on each side toward the margin of the blade, each again branching repeatedly and connecting with its neighbor, and so forming a network or skeleton of woody fibres which strengthen and support the leaf. These ribs are called veins or nerves,
and the whole is the "venation" of the leaf. Such a simple leaf (for example of the beech) is called reticulate or net-veined. In a large class of cases, however, the branches of the midrib do not spring at approximately equal intervals along its length, but all diverge from a point near its base, making a palmately veined arrangement, as in the maple. This reticulate veining is characteristic of dicotyledons. In another very distinct type of venation, char acteristic of monocotyledons, there is no mid rib, but the stalk divides at the base of the blade into many equal veins which extend in a more or less curving line through the length of the leaf, converging at the apex; such a leaf is said to be parallel-veined, as in grasses. Upon the plan of the skeleton depends mainly the form of the leaf, of which a great number of variations are named in botanical manuals and used in descriptions of species, depending mainly on the character and extent of the in dentation or incisions.
The arrangement of leaves upon the plant is an important matter. That it follows certain regular plans is apparent in buds, which when cut across exhibit their young leaves packed together in one or another of certain definite ways; and their relative position on the stem of an herb or the twig of a tree follows as a result of the law of growth in that group. The theo retical perfection of arrangement, however, is often greatly disturbed by the interference of older leaves with the development of the younger, and by other causes affecting the un symmetrical growth of the whole plant. The arrangement of leaves upon the stem, called phyllotaxis, is in most cases one of alternation, thus securing the uninterrupted exposure of the upper surface of the leaf to the sun. It is to obtain this exposure that plants struggle to become tall and bear their leaves most pro fusely at the summit; and that the branches of trees reach outward as far as possible; and the lower early leaves of many soon die off because shaded by the later, higher growth. The arrangement is carried out in two prin cipal ways: the leaves are either alternate, one after another, only a single leaf arising from each node or joint of the stem; or opposite, when there is a pair of leaves on each joint of the stem; but sometimes the leaves are whorled or verticillate, there being three or more in a circle on one joint of the stem. The result of this arrangement in an alternate-leaved stem is to cause the leaves to follow one another up the stern in a spiral manner; while any two successive leaves on the same species will also be separated from each other by just and equal portion of the circumference of the stem. The same principle governs the parts of the flower in which the sepals of the calyx typically alternate with the petals of the corolla, the petals with the stamens and the stamens with.