338 VEGETABLE PHYSIOLOGY 
secured by the turgescence of the parenchymatous cells, 
and buoyancy is much assisted by the air in the lacune. 
The primary root is generally feebly developed, and, as 
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. 
3 
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FaHLD 
Fia. 140.—Srcrion or Lear or [se8tes, 
a, lacunar cavities; 6, vascular bundle. 
The epidermis of both stem and root is not cuticularised, 
and therefore the cells remain capable of absorbing the 
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 
