10 THE STORY OF PLANT LIFE 
distinct regions for root, stem, etc. The arrange- 
ment of the leaves upon the stem is adapted to the 
exposure of a large surface to the light and action of 
the sun’s rays, and the conveyance of moisture to the 
roots, necessitating mosaic patterns, etc. The roots 
are necessarily adapted to terrestrial conditions, 
hence the different modifications of tap-root, fibrous 
root, rhizome, etc. These are a few features which 
are connected with the growth of plants on land. 
In water the conditions are entirely different. 
_ There is no need for a rigid, erect stem, or for dis- 
tinct regions for root, stem, etc. The habit is 
streaming, especially in the case of running water, 
the plant being limp and flaccid. The stems do not 
attain the dimensions or structure of trees, but are 
herbaceous, usually dying down in winter, though 
nearly all are perennial. But some have resting 
buds, or hibernacula enabling them to persist during 
the winter without dying down. The leaves are 
narrow, thread-like, exposing more surface propor- 
tionally to the light, but not continuously. There is 
no arrangement to convey water, but the upper and 
lower surfaces are modified to face the light, whilst 
their existence in water does not require of them the 
same conditions as land plants for transpiring or 
respiring. And the necessity of obtaining salts from 
the water introduces new conditions, though these do 
not affect habit. | 
Between these extreme types come the non-erect, 
trailing or prostrate plants that rely upon a flat sur- 
