Ecological Types of Stems 



175 



part of the plants. Thus plants growing on sandy bottoms make 

 poor growth when compared with the same plants growing on 

 rich humus-covered bottoms in the same lake. In submerged 

 plants there is a definite movement of water through the water- 

 conducting tissue just as in land plants. At the upper ends of the 

 leaves the water is given off through water pores. 



Emersed hydrophytes. A third group of hydrophytes, like 

 the cat-tails, rushes, bulrushes, and sedges, may have their roots, 

 stem bases, and underground stems under water, while the upper 

 parts are exposed to the air. These plants have both the con- 

 ductive and mechanical tissues well developed and are therefore 

 able to grow erect without being supported by the water. The 

 stems and leaves are exposed to the action of wind and wave and 

 to the conditions that bring about normal transpiration. 



The importance of the aerial portions of these plants and the 



Fig. ioi. Water lilies. In the foreground, species of Castalia; near the middle, the giant 

 water lily of the Amazon {Victoria regia). Stomata are found on the upper surfaces of the 

 leaves, and, as in land plants, there is a definite upward movement of water through the 

 plant, 



