THE PLAINS ASSOCIATION. 55 



ous course. The roots were dark brown with a very fleshy cortex, extremely 

 brittle, and when broken exuded a milky latex. At the top the diameter was 

 about 5 mm. As the root proceeded downward the diameter fluctuated from 

 4 to 7 mm., but gradually grew smaller to the tips, which were about 1 mm. 

 in diameter and ended abruptly. These roots are very different from the 

 much longer but almost unbranched roots of the same species growing in 

 loess soil (p. 16). 



Aragallus lambertii.— This loco unfortunately is a widely distributed legume 

 throughout the plains grassland, forming one of the most important of the 

 vernal societies. As is characteristic of most legumes, Aragallus has a strong 

 tap-root. At the top this was 9 mm. in diameter and with a long gradual 

 taper it penetrated the soil with a tortuous course almost vertically downward 

 to a maximum depth of 8 feet. For the first 4 inches of its course there were 

 scarcely any branches except 3 or 4 small hairlike laterals, which penetrated 

 horizontally for a distance of 2 or 3 inches. Below this point and to a depth 

 of 30 inches the tap was marked by rather profuse, small laterals, ranging 

 from hairlike to 0.7 mm. in diameter. These penetrated the earth in a more 

 or less horizontal direction for a distance of 2 to 8 inches; they were them- 

 selves branched to the second or third order. Below 30 inches the tap was 

 very sparingly branched throughout its course, these branches being of the 

 same character but much smaller than those already described. The tap 

 ended by dividing into 3 or 4 ultimate rootlets, 3 or 4 inches long. The root 

 was tough, rather woody, light brown in color. Three plants were excavated 

 and examined (fig. 16). 



Petalostemon purpureus.— This and the following subdominant have a 

 wide distribution throughout the grassland formation. Their presence in the 

 plains association is indicative of at least fairly good soil-moisture conditions. 

 They reach their best development in the prairies, where they form typical 

 societies. 



More than a dozen plants were examined. They have tap-roots varymg 

 from 3 or 4 mm. to 1 cm. in diameter. The general root system of all was 

 very similar. At a depth of 1 to 4 inches they invariably throw off 3 to 7 

 large laterals in a rather horizontal direction. These run off in the shallow 

 soil almost parallel with the surface and often not more than an inch below 

 it, to a distance of 8 to 18 inches before turning rather abruptly downward. 

 Usually 3 or 4 of these laterals are more pronounced than the others, some of 

 them being as large as 6 mm. in diameter. Like the tap-root, they taper very 

 rapidly and throughout their horizontal course give off both large and small 

 sublaterals. These are frequently minutely branched and compete with the 

 grasses for the water in the surface soil. Upon turning downward, these 

 laterals, now usually not more than 1 to 2 mm. in diameter, pursue a course 

 in general vertically downward, though more or less curved backward and 

 forward. The tap-root descends vertically or runs off 6 to 12 inches from 

 this direction like the laterals, and tapers so rapidly that at a depth of 1 foot 

 it is seldom more than 1 or 2 mm. in diameter. Below this level, all the roots 

 are more or less threadlike, branching profusely, especially with small 

 rebranched feeders varying in length from a few millimeters to several inches. 

 The depth of the lateral branches usually does not exceed 4 or 5 feet, although 

 some were traced to a depth of more than 6 feet. The maximum depth of a 

 tap-root 6 mm. in diameter was 5.5 feet, while another 9 mm. in diameter 

 reached a depth of 6.5 feet. Unlike the white prairie-clover, this plant is well 

 adapted to absorb the moisture in the surface soil, but like the former the roots 

 occur quite abundantly to a depth of 5 or 6 feet, through a cross-section of 3 or 



