220 



PLANT RELATIONS. 



the tumbleweecl habit is prominently developed. Certain 

 low and densely branching plants are lightly rooted in 



the soil, so that at the close of their gro 1 



period they 



are easily uprooted by the wind, and are rolled to great 



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

 ous thick and pointed, sword-like leaves. 



