ECADS. 85 



ended in the surface foot instead of turning downward. A similar habit of 

 other ecads when growing in sandy soil has already been pointed out (Weaver, 

 1919:110), and it will be shown that other species behave in this same way. 

 In the mixed prairies at Colorado Springs and at Ardmore the roots penetrated 

 a little deeper than at either Burlington or at Sterling, stations in the short- 

 grass plains. A comparison of the root habit of buffalo grass and grama with 

 that of wire-grass, especially with regard to lateral root distribution and 

 branching in the surface soil, makes it clear at once why the latter can develop 

 only poorly in undisturbed short-grass turf. Short-grasses begin to absorb at 

 once from the surface soil after summer showers; wire-grass only to a small 

 degree until water has penetrated more deeply. The root habit also explains 

 why wire-grass often thrives on short-grass land that has been broken out and 

 subsequently abandoned (c/. Shantz 1911: 41). 



Psoralea tenuiflora. — This widely distributed legume occurs in nearly every 

 association of the grassland formation. It is not only one of the most impor- 

 tant herbs of the short-grass plains, but is also abundant in the mixed prairie. 

 In less arid grassland, and especially in true prairies, it is represented by 

 extensive societies of the more robust form, Psoralea tenuiflora floribunda 

 (plate 5, b). 



Several plants of this species have been examined, both in the mixed prairie 

 at Colorado Springs and in the true prairie at Lincoln (Weaver, 1919). They 

 are characterized by strong tap-roots and numerous large, widely spreading 

 laterals, especially in the deeper soil. Frequently the tap-root penetrates to 

 depths of 1 or 2 feet before branching. Very little absorption occurs near the 

 surface. Maximum depths of penetration of 8 to over 12 feet have been 

 recorded. Although larger laterals may be quite abundant, these are only 

 poorly branched. 



A specimen with a tap-root an inch in diameter was excavated in the mixed 

 prairie at Limon. The crown of the plant, as is often the case, was sunk 6 

 inches below the soil-level. Two small laterals originated just below the 

 crown and two large ones at a depth of 1.3 feet. The latter were each 8 mm. in 

 diameter (plate 19, b). These ran off obliquely on opposite sides of the tap- 

 root, to distances of 4 and 12 inches respectively, before turning downward, 

 each pursuing a course almost parallel with the vertically descending tap-root 

 to a depth of 4.4 feet. At this level the tap-root gave off a lateral 5 mm. in 

 diameter and then turned off horizontally, running parallel with the soil 

 surface for 1.5 feet, but still retaining a diameter of 5 mm. Here it broke up 

 into two nearly equal branches, one of which ascended more than 6 inches, 

 while the other continued its course far into the wall of the trench. While no 

 roots were traced to a greater depth than 6 feet, several were found in different 

 trenches at this level which still maintained diameters of 4 or 5 mm., and from 

 their vertically downward course it is very probable that they penetrated 

 much deeper. 



In the short-grass plains at Yuma a specimen with a crown 15 mm. in di- 

 ameter was excavated from one end of the trench described on page 72. The 

 bushy plant was nearly 2 feet high and in full bloom. In figure 30 the details 

 of this root system may be seen. The absence of any laterals in the surface 

 1.7 feet of soil and the widely spreading but poorly branched deeper roots are 

 very characteristic. The tap was traced to a depth of 9 feet, at which point 

 it was still 4 mm. in diameter and running rather vertically downward. Plants 

 of this species were rather rare in the closed buffalo-grass sod (plate 9, a) . 



At Phillipsburg, Kansas, several psoraleas in the mixed prairie were traced 

 to the maximum depth of the trenches (about 8 feet) , but many of the larger 



