142 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



and air and probably also to differences in the amount and distribution 

 of nutrients. 



Aside from the relative differences in development in the two areas, 

 the rapid growth and great depth of root penetration of crop plants 

 should be emphasized. Using again the natural vegetation as the 

 indicator of conditions for agricultural practice, the rotation of short- 

 rooted and densely-rooted crops with those of longer and more spread- 

 ing root systems should prove advantageous, if, indeed, under intensive 

 agriculture two such crops might not be grown on the same area at the 

 same time (cf. Rotmistrov, 1914). 



It seems certain that under growth conditions of the less arid portions 

 of the grassland formation, many crop plants absorb both water and nu- 

 trients from the deeper soils, at least when they are approaching matur- 

 ity and often when the surface water-content is nearly or quite depleted. 

 We should keep clearly in mind that water and solute absorption at 

 different stages in the life of the plant does not depend upon the total 

 root-mass or surface, but is determined by the size of the functioning 

 parts. If the older roots in the surface soil become suberized or cu- 

 tinized, absorption would be confined to the younger, deeper portions. 

 This knowledge of root structure and function is quite essential in 

 selecting crops for drought resistance, as well as in the proper use of 

 water for irrigation, in determining different methods of tillage, the 

 application of fertilizers, and other farm practices (cf. Vorob'lev, 1916). 

 A determination of the actively absorbing portions of the root system 

 will throw much light upon the ability of a plant to adapt itself to ad- 

 verse environment. The extent and difficulties of the problem are 

 equaled only by their interest and importance. 



