SUMMARY. 147 



prairie species, whether in hard lands or sandhills, and is found only in 

 13 per cent of the species examined in short-grass plains. This cor- 

 relation between depth of root penetration and efficient rainfall in the 

 three plant associations is fundamental. 



Of the numerous factors affecting root development in the grassland 

 formation, water-content is the one of controlling influence. In "Ecolo- 

 gical Relations of Roots," page 110, it has been shown that the root 

 habit of polydemic species is usually profoundly modified when plants 

 are grown under distinctly different conditions, and that in general the 

 root position conforms strikingly with the distribution of water-con- 

 tent, although a few stable species were found which showed little or 

 no variation. Further studies of ecads show that practically all the 

 species examined, many under a wide range of environmental condi- 

 tions, revealed marked changes either in surface lateral spread of roots, 

 depth of penetration, or output of branches. These changes were corre- 

 lated in nearly every instance with changes in water-content of the soil. 



Soil texture exerts a direct influence upon root development, but 

 modifies it most profoundly through water-content and aeration. 

 Roots often show tortuous courses and modified branching in growing 

 through layers of hard soil, but no "hardpan" was encountered where 

 mechanical resistance was so great that roots could not penetrate. 

 Those of many species show a marked increase in their output of 

 branches when growing in less compact soil or upon leaving a compact 

 soil layer and entering a more mellow one, and especially upon entering 

 ancient burrows of earthworms or rodents. This difference is probably 

 largely due to increased aeration. In soils with a subsoil of alternating 

 layers of sand and clay, striking differences in branching occur. 



The general characters of the root systems of species are often as 

 marked and distinctive as are those of the aerial vegetative parts. Al- 

 though the root system may be profoundly modified when subjected to 

 different environmental conditions, it still retains the characteristic 

 impress of that species in its usual habitat. The root systems of 

 different species of the same genus, while often somewhat similar, may 

 be of entirely different types. 



Cereal crops grown at many stations in short-grass plains, mixed 

 prairie, and true prairie showed marked differences both in shoot and 

 root development. Root-growth was greatest in true prairie (100 per 

 cent), least in short-grass plains (51 to 79 per cent), and intermediate 

 in mixed prairie (80 to 95 per cent). Certain local variations were 

 directly correlated with an increased water-content. Similar correla- 

 tions of root and shoot development of native grasses have been de- 

 termined. Thus, crop plants, like the majority of native species, have 

 the most extensive roots in true prairie, where the subsoil is constantly 

 moist and the evaporating power of the air is not extreme. 



