INFLUENCE OF ENVIEONMENT ON PLANTS 331 



the turgescence of the parenchymatous cells, and buoyancy 

 is much assisted by the air in the lacunae. 



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 



FIG. 141. SECTION OF STEM OF Potamogeton, SHOWING AIR PASSAGES 

 IN THE CORTEX. 



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 

 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 



