THE GREEN LEAF 59 



Neither of these needs is specially pressing 

 in the case of water plants, and indeed we find 

 that when any of the descendants of the land 

 flora take to an aquatic life, they tend more 

 or less rapidly to lose those distinctive ana- 

 tomical characters that marked their terres- 

 trial forebears. Plants which are growing 

 submerged in water are obviously better 

 fitted to absorb it through any part of their 

 surface and consequently have less need for 

 elaborately specialised organs, either for 

 absorption or conduction, than those whose 

 roots alone are in contact with the damp soil, 

 while the rest of the body is exposed to the 

 drying influence of currents of air. 



At the same time the water plants also 

 escape most of the mechanical difficulties, 

 and easily maintain a properly spread out leaf 

 surface, and even an upright position, owing 

 to the circumstance that their specific gravity 

 is so nearly identical with that of water, 

 Their weight thus becomes an almost negli- 

 gible factor, especially as they are often 

 buoyed up in the water, owing to the 

 presence of air or gases entangled in their 

 tissues. 



But most of the higher water plants have 

 not entirely lost the traces of their terrestrial 

 inheritance. Even the roots of many of them 

 still function as absorbing organs, and the 

 mechanical tissue is often present, though in a 

 more or less rudimentary condition. Some- 

 times, indeed, as in species that inhabit 



