stems and leaves are an advantage, because the surface exposed to evaporation is smaller in proportion than it would be in leaves of the ordinary form. This is, I believe, the reason why succulent leaves and stems are an advantage in very dry climates, such as the Canaries, Cape of Good Hope, etc.
The genus Lathyrus, the wild pea, contains two abnormal and interesting species, in which the foliaceous organs give the plant an appearance very unlike its congeners. Fig. 30 represents L. niger, with leaves of the ordinary type. In the yellow pea (L. aphaca, Fig. 31), the general aspect is very different, but it will be seen on a closer
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| Fig. 30. | Fig. 31. | |
inspection that the leaves are really absent, or, to speak more correctly, are reduced to tendrils, while the stipules, on the contrary, are, in compensation, considerably enlarged. They must not, therefore, be compared with the leaves, but with the stipules of other species, and from this point of view they are of a more normal character, the principal difference, indeed, being in size.
The grass pea (L. nissolia, Fig. 32) is also a small species. It lives in meadows and the grassy borders of fields, and has lost altogether, not only the leaves, but also the tendrils. Instead, however, of enlarged stipules, the functions of the leaves are assumed by the leafstalks, which are elongated, flattened, linear, ending in a fine point, and, in fact, so like the leaves of the grasses among which the plant lives that it is almost impossible to distinguish it except when in flower. For a weak plant growing among close grass, a long linear leaf is, perhaps, physically an advantage; but one may venture to suggest that the leaves would be more likely to be picked out and eaten if they were more easily distinguishable, and that from this point of view also the similarity of the plant to the grass among which it grows may also be an advantage.
