468 



PLANT DISTRIBUTION 



(Chap. XII), most roots are sensitive to moisture, grav- 

 ity, and dissolved salts in the soil, and respond to 

 them in a way to secure a favorable position. A cactus 

 root, however, must already be in position if it is to 

 take advantage of the occasional showers. This is 

 brought about through the response of the root to heat. 

 Responding positively to a warm temperature keeps the 

 roots in the upper and warmer layers of soil where the 

 water will be when it does come. Certain other desert 



Fi<i. l.'Jo. — Chaparral Vcj?Ptation on the inland side of one of the 

 California ('oast KanROs. The plants are mostly everRreen shrubs of 

 manzanita, small oaks, and (.eanothus. Where local conditions 

 are favorable tree growth is well developed. Live oak, California 

 bay, and a few other broad-leaved evergreen trees, as well aa a few 

 deciduous trees, make up the tree growth in the picture. Dryer 

 hills in Southern California have only the shrubs, and these are 

 smaller and .separated. (Crandall.) 



plants grow where there is subterranean water to be 

 had by sending enormously long roots down to it. 



Crr APARRAL (Fig. 135) is a name of a vegetation type 

 exhibited in groat perfection in California and in parts of 

 the Mediterranean region of Europe. It occurs in regions 



