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THE STRUCTURAL CHARACTERISTICS OF THE COMMUNITY 43 



potent factor in causing the layering. Weaver (1920, 1925) has 

 demonstrated many cases of the influence of soil moisture, soil granula- 

 tion, and distribution of nutrients upon root layering. Plants grown 

 in the same soil but with different amounts of precipitation will show 

 quite different root systems (Fig. 23). 



The roots of some plants are very sensitive to light. Many are able 

 automatically to regulate the growth of rhizomes according to illumina- 



T . 

 Stem heighf 



Fig. 23. — Root development in winter wheat under conditions of different annual 



rainfall. {After Weaver.) 



tion. Raunkiaer showed that the rhizome of Polygonatum multi- 

 florum, on reaching the normal distance from the surface of the ground, 

 grows horizontally forward but that if it is covered more deeply for 

 protection, it grows obliquely upward. 



The effect of aeration upon the course of roots is well known. In 

 wet, poorly aerated soils the root systems are poorly developed and the 

 roots tend to be spread out horizontally in the uppermost layer as in 

 the pines and birches in high moors. 



Soil acidity acts similarly when an alkaline calcareous soil is covered 

 with a layer of acid humus. The roots of acidophilous plants which 



