ON LEAVES. 



487 



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, tbe reason why succulent leaves 

 and stems are an advantage in very dry climates, such as the Canaries, 

 Cape of Good Hope, etc. 



The genus Lathy rus, 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 



Fia. 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 prin- 

 cipal 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 en- 

 larged stipules, the functions of the leaves are assumed by the leaf- 

 stalks, 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 sug- 

 gest 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 che plant to the grass among which it grows 

 may also be an advantage. 



