PRAIRIE PLANTS OF SOUTHEASTERN WASHINGTON 289 



small leaf surface of this knot-grass, it has always been a puzzle 

 to the writer how the plant could keep alive and even thrive and 

 blossom abundantly from July to October upon the parched soils 

 of the rim-rock. 



DISCUSSION AND CONCLUSIONS 



The most obvious result of a consideration of these data is the 

 fact that the response of the plant to these severe environmental 

 conditions is met by well-developed and extensive root-systems. 

 For just as the evaporating power of the air and the nature of 

 the transpiring organs determine the water-requirements of 

 plants, likewise the soil water and the nature of root-systems 

 determine the water supply. It is interesting to note, however, 

 that while the condition of light summer rainfall is quite un- 

 favorable for shallow-rooted grasses, still three of the most 

 important prairie grasses are shallow-rooted. Of these Poa 

 sandbergii is the first to appear on the thin soils of the rim-rock 

 and scab-land, where the underlying rock is little broken. In 

 the prairie it is an important interstitial plant. It is only in the 

 deep soils of the prairies that the shallow-rooted Koeleria cristata 

 and Festuca ovina ingrata play their important role. The water- 

 retaining capacity of the soils certainly favors these shallow- 

 rooted species. On the other hand, the deep-rooted Agropyron 

 spicatum early assumes importance as a crevice plant where 

 sufficient cleaving of the rock has occurred. Indeed, from then 

 on it becomes dominant in the bunchgrass association, where 

 the soils of only a few inches depth overlie more or less decom- 

 posed basalt. In the prairie it is one of the facies, but under 

 the new substratum conditions it partly abandons the bunch 

 habit and may become more or less of a sod former. However, a 

 discussion of the relation of root-systems to succession had best 

 be considered at another time. 



While the purpose of this study was not to arrive at a classi- 

 fication of root-systems, it is instructive to note that according 

 to the types of root-systems as set forth by Cannon, 8 all the roots 



8 Cannon, W. A. The Hoot Habits of Desert Plants, Carnegie Inst. Wash. 

 Publ. 131. P. 87, 1911. See also Von Alten, Hermann, Wurzel-studien. Bot. 

 Zeit. 67: 175-200, 1909. 



LIBRA 



V- 



