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 



FIG. 140. SECTION OP LEAF OP Isoetes. 

 , lacunar cavities ; b, 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 



