New Zealand Plants and their Story
Adaptations of the Heath Plants
Adaptations of the Heath Plants.
Without water, plants cannot exist. It may therefore be expected they have developed many special contrivances to insure the necessary supply. Especially important are those connected with water storage or saving, where there is danger of drought. The clay hills on which the northern heath flourishes, although soaked with water in the winter, become exceedingly dry in the summer, and had the plants no provision for husbanding; their water they would die for lack of moisture.
The leaves of plants serve various purposes. On their undersurfaces usually are many most minute openings, too small by far to be seen with the naked eye, which afford communication between the interior of the leaf and the atmosphere. Through these openings a constant stream of water-vapour issues, and through them, too, a perpetual current of air enters. From the air the plant gets its oxygen for breathing, and its carbonic acid for food purposes, while the water passing away makes room within the plant for a fresh supply charged with nutritive matter drawn from the soil by the roots. If, page 55however, the water-vapour passes from the leaves faster than it can be replaced by the roots, the former will wilt, and in time the plant would die of thirst.
Many contrivances have been evoked to hinder this, and the form of the leaves is in part an expression of the relation of a plant to its water-supply. The heath and the subalpine-scrub plants noted below show many interesting drought-resisting contrivances. Although a leaf loses water principally through its minute pores, some may pass away from the whole leaf-surface. To hinder this the surface is specially thickened, or covered with water-resisting substances, such as wax.
Variations of leaf-form play an important part. Do away with leaves altogether and the case is met. Carmichaelia australis, a northern plant of the pea family, is leafless, but has green, flattened stems, which can function as leaves. The almost leafless wild-irishman (Discaria toumatou) has developed rounded spines which, though they serve likewise as leaves, offer much less surface than would long green stems. The needle-like leaves of the dracophyllums not only present little surface, but they are vertical, and so never feel the full effect of the sun's rays, which together with dry air and high winds have much, effect on the rapidity with which a plant loses water. Pomaderris phylicaefolia reduces its water-losing surface by recurving the edges of its leaves. Olearia furfuracea has a mat of dense hairs on the under-surface of each leaf.