30 THE BIOLOGY OF FLOWERING PLANTS 



The lateral spread in the surface regions is always small. 

 Lower down the roots may spread through a radius of 

 4 ft. from the centre of the system, though generally the 

 spread is less than this, and frequently less than i ft. 



The great depth 

 usually attained is only 

 possible in the very 

 deep soils of the 

 prairies : " The fertile 

 dark-coloured prairie 

 soil of the region is 

 the type commonly 

 called loess, much of 

 which, however, is con- 

 founded with glacial 

 drift. The loess covers 

 the hills and valleys 

 alike to a depth of 

 from 20 to 100 ft., 

 being much thicker 

 than this in places 

 and much thinner in 

 others." The region 

 has an annual rainfall 

 of nearly 30 in., most 

 of it in the growing 

 (summer) months ; but 

 there is much run-off 

 in the heavy storms. 

 This, coupled with 

 high evaporation, pre- 

 vents the surface 

 regions from retaining 

 much moisture. Fre- 

 quently there is little 



Fig. I, — Root system of Liatris punctata, 

 from the prairies of Nebraska. (From 

 Weaver.) 



water available for plant growth in the first 5 ft. of soil. 

 This accounts for the deep penetration of most root 

 systems. The shallow-rooted grasses complete their active 



