STEMS 



665 



trees, as Gleditsia, they are more numerous on the under side of horizontal branches, 

 while occurring equally on all sides of vertical branches. 



Carbohydrate synthesis in leafless stems. From the standpoint of 

 synthesis the most important stems are those on which leaves are insig- 

 nificant or wholly absent, for here the stem becomes the chief food- 

 making organ. This habit is well illustrated by the cacti, whose stems 



FlG. 976. A colony of bulrushes (Scirpus validus) ; though the habit of an individual 

 shoot is that of an extreme xerophyte well fitted for protection from intense light, a crowded 

 colony of such shoots is fitted for optimum lighting; Selkirk, Manitoba. Photograph 

 by E. W. COWLES. 



may be flattened (Opuntia, figs. 1040-1042), cylindrical (Cereus, fig. 

 1035), or spherical (Echinocactus, fig. 1063); the stem of Echinocactus 

 with its small surface in proportion to its volume represents the extreme 

 antithesis of thejorest mesophyte with its expanded leaves. Such stems 

 contrast with ordinary leaf-bearing green stems in possessing strong 

 palisades in place of weak palisades or sponge tissue. The chlorophyll 

 also extends much deeper than in most stems and leaves, reaching the 



