SOIL AND SUBSOIL. 



171 



Not unfrequently the entire root system developed by the up- 

 permost bud perishes; but usually its main roots soon begin to 

 recede from the threatening drought and heat of the surface, 

 curving, or branching downward in the direction of the moist- 

 ure supply, and without detriment to their nutrition because of 

 the practical identity of the surface soil and subsoil. As the 

 portions of the roots near the surface thicken and mature, 

 their corky rind soon prevents their being injured by the arid 

 conditions to which they are subjected; while the root-ends, 

 finding congenial conditions of nutriment and aeration in the 

 moist depths, develop without difficulty as they would in their 

 humid home. Practically the same process of adaptation takes 

 place in every one of the trees, shrubs, or perennials belonging 

 to the humid climates, until their root system has assumed 

 nearly the habit of the corresponding native vegetation. 



The photograph of the roots of a hop plant, grown on bench 

 lands of the Sacramento river, shows the roots extending to 8 

 feet depth, but where broken off the main root is still nearly 

 two millimeters in thickness, proving that it penetrated at least 

 two feet beyond the depth shown in figure 31. 



In the case of native annuals, either the duration of their 

 vegetation is extremely short, ending with or shortly after the 

 cessation of rains; or else their tap roots descend so low, and 

 the nutritive rootlets are developed at such depth, as to be be- 

 yond reach of the summer's heat and drought. For while it is 

 true that rootlets immersed in air-dry soil may absorb plant- 

 food, this absorption is very slow and can only be auxiliary to 

 the main root system which, instead of terminating in the sur- 

 face soil as in the humid region, will be found to begin to 

 branch off at depths of 15 and iS inches, and may then in sandy 

 lands descend to from 4 to 7 feet even in the case of annual 

 fibrous-rooted plants like wheat and barley. 1 In the case of 

 maize the roots of a late-planted crop may sometimes be found 

 descending along the walls of the sun-cracks in heavy clay land 



1 Shaler (Origin and X.itun- of Soils; uth Kept. I". S. Ceol. Survey. P. 31 I ) 

 s;iys " Annual plants cannot in their l>rief period of growth push their roots more 

 than six to twelve inches l>elo\v their root-crowns " a generalization measurably 

 true for the humid region only. According to F. J. Alway, the root> of cereals 

 penetrate to 5-7 feet in Saskatchewan, also. 



