INFLUENCE OF ENVIRONMENT ON PLANTS 331 
a rule, does not persist through the life of the plant. The 
floating forms frequently have no roots, but in many cases 
adventitious roots are given off in large numbers from the 
various nodes of the stem. The root-hairs*which are: so 
characteristic of terrestrial roots are usually either very 
scanty or altogether absent. 
The epidermis of both stem and root is not cuticularised, 
and therefore the cells remain capable of absorbing the 
Fie. 140.—Srcrion or Lear or Isoétes. 
«, lacunar cavities; 0, vascular bundle. 
water in which the plant is living. In the stem this tissue 
very frequently contains chloroplasts. 
The character of the leaves differs according to the 
habitat. Those which grow in rapid streams are generally 
either long and thin, or are very much, and finely, divided, so 
that they offer, in either case, no resistance to the force of 
the current. In more sluggish water they may be long 
and ribbon-like, but are frequently broader, and sometimes 
attain a considerable size. The cell-walls of the former 
are often thickened, but in the-latter the tissue is always 
very weak, the parenchyma of the mesophyll sometimes 
