220 



PLANT KELATIONS. 



the tumble weed habit is prominently developed. Certain 

 low and densely branching plants are lightly rooted in 

 the soil, so that at the close of their growing period they 

 are easily broken off by the wind, and are rolled to great 



Fig. 194. Tree-like yuccas from the arid regions of Africa, showing the very numei 

 ous thick and pointed, sword-like leaves. 



