166 



PLANT PHYSIOLOGY AND ECOLOGY 



The development of air passages for aeration is great in amphibious 

 and floating forms, but they are normally absent from submerged 

 plants, in which they persist occasionally as vestiges. Submerged 

 plants grow in light that is more or less diffuse, owing to the 

 absorption of rays by the water, and their photosynthesis is much 

 like that of shade plants, while the other forms are sun plants. 

 The fibrovascular system, which is only moderately well developed 

 in the amphibious type, is considerably reduced in floating plants, 

 and is little more than a remnant in submerged ones. 



185. Amphibious plants. The species of this group are closely 

 related to mesophytes: they are the least specialized of water 



Fig. 55. An amphibious plant, the white marsh-marigold, Call ha lepto- 

 sepala. The floating form with long petioles was produced by the 

 change of the marsh into a lake. 



plants. As a rule, they grow in saturated soil or in shallow water. 

 Owing to their frequent occurrence at the water's edge, many 

 amphibious plants have a wide range of adjustment, and may 

 grow for a time as mesophytes, or partially submerged. In the 



