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 ina 
more or less rudimentary condition. Some- 
times, indeed, as in species that inhabit 
