Ecological Groups of Stems 157 



Other hydrophytes, like the pondweeds and water lilies, 

 are rooted in the soil, and their stems bear submerged or 

 floating leaves. The stems have little or no mechanical tis- 

 sue. As compared with land plants, the conductive system 

 is much reduced. Many of these hydrophytes develop 

 underground rootstocks and tubers. For this reason the 

 plants commonly grow in masses. 



A third group of hydrophytes are those like the cat-tails, 

 rushes, bulrushes, and sedges, whose roots and stem bases 

 may be under water, while the upper parts are exposed to the 

 air. These plants have both the conductive and mechanical 

 tissues well developed. This is in keeping with the fact that 

 such plants are exposed to the action of wind and wave and 

 to the conditions that bring about normal transpiration. 



The most distinctive feature of submerged stems is the 

 presence of large air chambers extending throughout their 

 length. When the stems are broken open, the tissues are 

 seen to occupy much less space than the air cavities. We may 

 properly speak of " intercellular spaces " inmesophytic stems ; 

 in describing hydrophytes, the term " air cavities " is more 

 appropriate. They buoy up the plant and provide an internal 

 atmosphere for gas exchanges between the leaves and roots. 



Stems of xerophytes. The xerophytes are the character- 

 istic plants of deserts and dry plains. They occupy sand 

 dunes and sand plains along the Atlantic coast and on the 

 shores of the Great Lakes. They may be found locally on 

 rock cliffs and on dry, exposed hilltops, situations in which a 

 reduced water supply in the soil is accompanied by atmos- 

 pheric conditions that promote rapid transpiration and in 

 which the plants are periodically or continuously subjected 

 to drought. Plants that thrive in these habitats show a re- 



