CHAPTER VI 



SPECIAL FUNCTIONS AND FORMS OF STEMS 



82. Stems not always for the support of leaves. In Chap- 

 ter V considerable space was given to explanations of some of 

 the ways in which stems serve to support leaves in advan- 

 tageous positions for photosynthetic work. This is not, how- 

 ever, by any means the only function of the stem or portions 

 of the stem. Something was said in Chapter IV about the 

 importance of food storage 



in stems, and something in 

 section 56 about their work 

 of photosynthesis. Some 

 other important things done 

 by whole stems, or by more 

 or less highly specialized 

 portions of them, will be 

 discussed in the present 

 chapter. 



83. Storage of air or 



water in the stem. It sel- Fl - 68< Cross section of 8tem of P nd - 



weed (Potamogeton). showing air passages 

 dom happens that the same 



a, much magnified. After Green 



plant would be benefited 



by storing both air and water abundantly in its roots or stems ; 

 but marsh and water plants may evidently be much aided in 

 their respiration by holding a good deal of air inside the plant 

 body. In some such plants the air passages and air cavities 

 form a complex system, which extends all the way from the 

 stomata to the tips of the roots. A section across the stem of 

 any of the common pondweeds (Potamogetoii) (fig. 68) shows 

 clearly how large and abundant its air passages are. 



85 



