Ecological Types of Stems 



173 



features of the hydrophytes, about to be described, must not be 

 considered as necessarily due to the direct influence of the envi- 

 ronmental factors to which they are exposed. 



Effect of submergence on mesophytes. When mesophytes are 

 grown under very moist conditions, one of the first changes ob- 

 served is the great increase in air spaces among the cells. Fur- 

 thermore, when stems of mesophytes are submerged they may 

 develop large air cavities by the breaking down and separation 

 of certain groups of cells in the cortex. While the stems conse- 

 quently increase in diameter, there is a decrease in the amount 

 of wood tissue. Certain terrestrial plants with heavy cuticle and 

 hairy coats are smooth and without cuticle when grown under 

 water (Fig. 5). 



Stems of hydrophytes. The most distinctive feature of sub- 

 merged stems of hydrophytes 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 " in 

 mesophytic stems ; but in describing hydrophytes, the term '' air 

 cavities " is more appropriate. These air cavities buoy up the 

 plant and provide an internal 

 atmosphere in which gas ex- 

 changes between the leaves and 

 roots take place. Frequently 

 the cells are so distinctive in form 

 and arrangement that we have 

 as a result a special tissue which 

 is called aerenchyma. Living 

 cells of plants are slightly heavier 

 than water, and the ability of any 

 plant to float is due to the air con- p^^ ^^ ^.^ ^^^.^^^ ^^^ ^.^^^ ^^^^ ,^ 



tained in the intercellular spaces, petiole of yellow water Uly {Nymphaa). 



Floating type. Many hydro- The most characteristic feature of the stems 



*=• "^ ^ ./ -' of hydrophytes is the presence of large air 



phytes are free-floating. Among spaces in the tissues. {After Frank.) 



