18 THE ECOLOGICAL RELATIONS OF ROOTS. 



and nodules at a depth of 16.5 feet. The roots are smooth and chocolate- 

 brown in color, all being rather tough and the older parts distinctly woody. 

 Here again it should be noted that the plants growing in the mellow loess 

 reached depths 4 or 5 feet greater than those examined in the clay-loam soil 

 at Belmont. 



PRAIRIE ROOT SYSTEMS AND THE PRAIRIE ENVIRONMENT. 



The most obvious conclusion from a consideration of these data is 

 the fact that prairie species are provided with well-developed, deep- 

 seated, and extensive root systems. Upon the basis of root depth 

 the 33 species examined may be divided into three groups : 



1. Shallow-rooted plants are those that seldom extend below the 

 first 2 feet of soil. These consist wholly of grasses, such as Kceleria 

 cristata, Stipa spartea, Elymus canadensis, Distichlis spicata, Sporoh- 

 olus longifolius, and Aristida oligantha. 



2. Plants with roots extending well below the second foot of soil but 

 seldom deeper than 5 feet may be grouped as intermediate in root 

 depth. Here belong Andropogon scoparius, A. nutans, Bouteloua gra- 

 cilis, Bulbilis dactyloides. Verbena stricta, Helianthus rigidus, Solidago 

 rigida, and Petalostemon candidus. Here also may be placed Grindelia 

 squarrosa, a plant which sometimes extends beyond this depth. 



3. Of the plants studied, 55 per cent have roots which extend beyond 

 a depth of 5 feet ; indeed, most of them to depths of from 7 to 9 feet and 

 a few to a maximum depth of from 13 to 20 feet or more. These may 

 be classed as deep-rooted species. Here belong Panicum virgatum, 

 Andropogon furcatus, Agropyrum repens, Solidago canadensis, Liatris 

 pu7ictata, Silphiuni laciiiiatum, Amorpha canescens, Astragalus crassi- 

 carpus, Psoralea tenuiflora, P. argophylla, Lygodesmia juncea, Ceanothus 

 ovatus, Baptisia bracteata, Lespedeza capitata, Glycyrrhiza lepidota, 

 Brauneria pallida, Vernonia baldwinii, and Kuhnia glutinosa. 



To understand the causes for such remarkable root development 

 it will be necessary for us to study the prairie environment. The 

 prairies of eastern Nebraska receive more moisture than most of the 

 great grassland area. The mean annual precipitation for Lincoln, 

 together with its seasonal distribution (which is of greater ecological 

 significance), is shown in table 1. 



It may be seen that most of the precipitation falls during the grov/ing 

 season and less than one-tenth of it during the three winter months. 

 About half of the rainfall of May, June, and July is from rains of an 

 inch or more in 24 hours. Such a seasonal distribution of moisture is 

 very favorable for the growth of grasses. Not infrequently, however, 

 storms occur with a rainfall exceeding 2 inches and occasionally 4 or 5 

 inches in a period of 24 hours. Such storms invariably result in a high 

 run-off and they account largely for the observed deficiencies of mois- 

 ture for crops in seasons where the recorded rainfall would indicate an 

 abundant supply. Drought periods of 30 or more consecutive days 

 between March 1 and September 30, in which precipitation to the 



