98 



PLANT LIFE. 



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

 larly circular outline (fig. in). Occasionally the sur.ace 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 free/ when 

 enough mechanical tissue is developed to render them suffi- 

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



'1 *■ "•,.'■■ 

 . - i -'• . 





A 



Fig. iio. — Cactuses, showing form. A, Ct'rt^ns dasyacantJius. /?, F.clii nCiact us 

 hoi-i7,o iilaiis. Jn both the clusters of spines arise from tnbercles on tlie stems. 

 Reduced. — After fvei^ner 



tlie 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 crecphif;, stems and the erect form every 

 conceivalile position exists. The direction of growth is deter- 

 mined largely by the relation of the ]ilant to gravity and light 

 as stimuli. (See "i*! 2S5, 2<S7.) Other stems rise into the 



