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- 
dom happens that the same 
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 (Potamogeton) (fig. 68) shows 
clearly how large and abundant its air passages are. 
85 
Fic. 68. Cross section of stem of pond- 
weed (Potamogeton), showing air passages 
u, much magnified. After Green 
