93 



PLANT LIFE. 



122. A section of the stem commonly presents an irregu- 

 larly circular outline (fig. in). Occasionally the surface of 

 the stem is fluted or channeled, and, if these grooves or 

 channels be few and the corresponding angles prominent, the 

 section of the stem is polygonal, with three, four, five, six, or 

 more sides. 



123. Habit. — As to habit, stems are commonly erect when 

 enough mechanical tissue is developed to render them suffi- 

 ciently rigid to carry not only their own weight, but that of 



Jmw^ 



A B 



Fig. iio. — Cactuses, showing form. A, Cereus dasyacanthus. B, Echinocactus 

 horizontalis. In both the clusters of spines arise from tubercles on the stems. 

 Reduced. — After Kerner. 



the leaves and other members attached to them. Other stems 

 lie flat upon the ground, to which they may or may not attach 

 themselves by the development of secondary roots. Between 

 these prostrate, or creeping, stems and the erect form every 

 conceivable position exists. The direction of growth is deter- 

 mined largely by the relation of the plant to gravity and light 

 as stimuli. (See ■ •; 285, 287.) Other stems rise into the 



