Читать книгу Flowers of the Coast - Ian Hepburn - Страница 17

Passing now to the other end of the plant, we must say something about the process by which the surplus water is disposed of at the leaves. This is known as transpiration. On the surface of any leaf a number of minute pore-like openings are to be found which are called the “stomata.” Each stoma usually takes the form of a slit between two elongated cells known as “guard cells,” lying side by side (Fig. 3(c).) The opening or closing of the pore is controlled by the swelling or contraction of this pair of cells. Thus, when the turgor pressure of the plant is high, the cell-walls expand and the slit is opened to aid the elimination of water. When the water-supply is less abundant, the turgor pressure falls and the cells contract so that they lie with their walls in contact with each other, thus closing the slit. It should be emphasised that the stomata are not only concerned with the elimination of water-vapour but are also the organs through which the plant absorbs carbon dioxide and gives out oxygen in the carbon assimilation process (photosynthesis). They are in fact the openings through which the exchange of all gases takes place, although to some extent the whole surface of the leaf and even the stem functions in this capacity. When the external covering or “cuticle” of the leaf is thick, however, the process is largely confined to the stomata. Usually these occur more thickly on the under-surface of the leaf, as being better protected from the drying influence of the sun. Only in water-plants with floating leaves are they confined to the top surface. Although the number of stomata found on the leaves of different plants varies greatly, there do not appear to be any fewer on those belonging to halophytes or xerophytes than on the leaves of normal plants. A moderately large leaf with an average density of stomata may possess several millions of such openings.

Although there is much which is obscure about the transpiration process, it has two important effects. Firstly, it maintains a constant flow of water from root to leaf through the wood of the plant, bringing with it also small quantities of dissolved salts which are essential for the plant’s growth. Secondly, it tends to reduce the temperature of the leaf when it is exposed to the heat of the sun. It is a well-known fact that when a liquid is changed into vapour, energy (latent heat) has to be expended. This heat is derived from the air immediately in contact with the surface of the leaf and in this way the leaf itself is cooled. In hot climates and in dry habitats this result may be important. The chief danger with xerophytes, and to a lesser extent with halophytes, is that the loss of water by transpiration may be so rapid that it cannot be replaced from the scanty supply of water available at their roots. Many plants belonging to both these classes are therefore equipped with devices to check excessive transpiration, and some of these will now be described.