24 THE ECOLOGICAL RELATIONS OF ROOTS. 



The roots of prairie plants are grouped into more or less definite 

 absorbing layers, many of the deeper-rooted species having few or 

 no absorbing roots in the first few feet of soil. This layering of the 

 roots reduces competition and permits the growth of a larger number 

 of species. Any of the shallow-rooted grasses extract water from the 

 soil layers quite above those from which Andropogon furcatus or 

 Solidago rigida draw their supply, while still deeper soil strata furnish 

 moisture for such deep-seated species as Panicum virgatum, Silphium 

 laciniatum, and Kuhnia glutinosa. It should be pointed out here that 

 prairie plants very seldom show the widely spreading and superficial 

 laterals so characteristic of many plains species. The cause seems to 

 lie in the greater water-content of the deeper soil in the prairie. 



Root variations of prairie plants may be due to a number of factors, 

 among which the water-content of the soil and its penetrability prob- 

 ably stand first in importance, at least in non-alkaline soils. Although 

 the problem of the causes of root variation is one which must be solved 

 under control conditions where one factor can be varied at a time, 

 considerable evidence has been found in the field that both soil mois- 

 ture and soil texture profoundly influence root distribution. 



The effect of compact soils upon root penetration is shown not only 

 by the tortuous courses pursued by roots and the distortions of the 

 roots themselves, but also by their modified branching. Several 

 species, including grasses, were found to penetrate from 2 to 3 feet 

 deeper in deep loam soil than in the same type of soil when it was 

 underlaid with a hard clay subsoil. In soils with a subsoil of alternating 

 layers of sand and clay a striking distribution of lateral roots was 

 observed. These often occurred abundantly near the bottom of the 

 sandy layers and in the clay strata where the latter acting as a rather 

 impervious layer had retained much soil moisture. 



It is interesting to note that species of the same genus may have 

 an entirely different type of root system. Liatris punctata has a 

 deep tap-root, while the multitudinous roots arising from the corm of 

 L. scariosa are fibrous. Like\vise, the roots of Andropogon scoparius 

 are as different from those of A. furcatus as are the roots of Kceleria 

 cristata from those of Stipa spartea. Indeed, the roots studied are 

 remarkable for their individuality. The roots of each species, because 

 of peculiarities of form, branching habit, position in the soil, texture, 

 color, odor, or taste, can be easily distinguished, and these distinguish- 

 ing characters have often proved useful in ecological work. In some 

 cases it has been possible to reconstruct successional changes from the 

 record left in the soil by partially preserved roots and rhizomes. 



