It has been seen that unless a plant is a parasite or saprophyte, using as food ready-made organic material, it is necessary that it should possess a sufficient expanse of green (i.e., chlorophyll-bearing) tissue for the purpose of assimilation. This is the essential function of the leaves; but before leaving the study of stems it should be pointed out that they usually assist, and sometimes entirely replace, the leaves as organs of food-manufacture. We have seen how in dry places—whether physically dry, from direct scarcity of water, or physiologically dry, owing to reduced activity on the part of the plant due to unfavourable conditions, such as obtain in cold regions, or on poisoned ground like salt-marshes or bogs—leaf surface tends to be reduced, to avoid excessive loss of water. In such plants as the Cacti, and the Euphorbias which so closely mimic the cactus form, this reduction is carried to its limit. Leaves are absent, and the stems, greatly swollen so as to store water, take up the process of assimilation, and perform it satisfactorily. In more rapid-growing plants, a sufficient area for assimilation may be obtained by abundant branching, as in the Gorse, in which leaves are present only in the seedling stage. In the Brooms (Genista) the leaf-development is often weak, but the stems sometimes make up for this by bearing green flattened wings. In the Spanish Broom (G. sagittalis), a straggling shrub inhabiting dry places in south-west Europe, the few ovate hairy leaves, produced in spring, soon fall; but the slender branches bear several (two to four) broad green wings, which act as
Fig. 20.—Genista sagittalis. 1/2.
leaves, and persist for a couple of years, when they pass away, leaving slender, round, brown stems. In our native Broom (Sarothamnus scoparius) a similar modification may be observed, though of less degree. Sometimes stem-structures assume a very leaf-like form, as in the Butcher’s Broom (Ruscus aculeatus), where the ultimate branches are ovate and quite flat, and might be taken for true leaves but for the fact that they bear on their surface flowers, and subsequently berries. The leaves themselves are in this plant reduced to minute scales, and from their axils these flattened branches spring. In fact, where leaf reduction takes place, the process of assimilation is often shared in varying degree by the leaves, the stipules, and the stems. Among our native plants, as, for instance, in the Leguminosæ and Rosaceæ, the reader may find for himself many interesting examples for examination.
But the large majority of the Seed Plants bear well-developed leaves, to which the process of assimilation is practically confined.
Leaves vary surprisingly in size, shape, and arrangement, features which are closely related to the characters of the stems which bear them, the object being the most advantageous display of the chlorophyll in relation to the light-supply. In general they naturally take the form of a broad thin blade, protected as may be necessary against extremes of weather, and guarded against the obvious danger of being dried up by a thin waterproof covering or cuticle outside the epidermal layer of cells. In leaves we find the same beauty of mechanical construction as is seen in stems. The problem is again that of securing maximum efficiency with minimum expenditure of material. To give as great a surface as possible, the leaves are as broad and thin as is consistent with safety, the question of damage by wind being an important controlling factor. The veins, or vascular bundles, act efficiently as strengtheners of the thin surface; to prevent tearing at the leaf-edges the veins are often looped along the margin; while in indented leaves the extremities of the indentations are strengthened with special tissue. When one surface of the leaf faces the sky, as in most cases it does, this surface is strengthened against the weather, and the stomata are arranged mostly on the lower surface. Where occasionally the leaves hang normally in a vertical position, as do the mature leaves of the Gum Trees (Eucalyptus), both sides are protected, and the stomata are borne on the two faces equally. In the Water Lily, again, whose leaves float, the upper face, which alone is exposed to the air, bears the stomata, which are present in unusual numbers—nearly 300,000 to the square inch; the leaf surface is toughened to resist rain and wind, and waxy to prevent water from lying on it and so interfering with transpiration. The presence or absence of a leaf-stalk, again, is often clearly related to the light question. In the Water Lilies the continued lengthening of the elongated petiole causes the older leaves to float clear outside of the younger ones. In many biennial herbs, where food is stored up during the first season in preparation for the flowering effort in the second, a similar arrangement prevails—note the leaf-rosettes displayed by Spear Thistle (Carduus lanceolatus) and Herb Robert (Geranium Robertianum), as also especially in winter by perennials like the Dandelion (Taraxacum officinale) and Ribwort (Plantago lanceolata). Where stems spread horizontally, as the lower branches of trees, the leaves are arranged more or less in one
Fig. 21.—Azara microphylla. 1/2.