THE PLAINS ASSOCIATION. 



59 



Fig. 18. — Ratihida columnaris. 



penetrated several feet further. In color the root is reddish brown and covered 

 with a scaly, shreddy cortex, the older parts being quite woody (plate 19, c) . 



Ratibida columnaris. — This species is widely distributed throughout the 

 grassland formation, where it forms summer societies dominating large areas. 

 It has a strong tap-root, 8 to 10 mm. in diameter. For the first 5 inches it 

 tapers very rapidly until at the end of 

 that distance it is not more than 3 mm. 

 in diameter. From that point it passes 

 vertically downward with a zigzag, 

 irregular course, very crooked and 

 crinkled, tapering slightly (at 12 inches 

 being 1.5 mm. in diameter) to a max- 

 imum depth of 2 feet. The first 4 or 5 

 inches is marked by an extraordinary 

 number of more or less horizontal 

 branches which vary in diameter from 

 0.2 to 0.5 mm.; these extend out 

 through the surface soil to a distance 

 of 6 to 12 inches and are themselves 

 profusely branched with small hairlike 

 rootlets from 0.5 to 2 cm. long. The 

 number of these primary horizontal 

 branches is very large; one count 

 showed approximately 13, while 

 another showed about 40. Below 4 or 



5 inches the root is seldom branched, 

 what branches there are being hairlike and from 1 to 3 cm. long. Near the 

 end, however, the tap usually branches into two or three small, short rami- 

 fying parts. The root is dark brown in color, rather tough and unyielding. 

 Four plants were examined (fig. 18). 



Senecio aureus oblanceolatus. — Senecio is a very common and often abun- 

 dant plains species, frequently forming vernal societies in the grassland. The 

 specimens examined were small plants 6 to 10 inches high. The multiple roots 

 are so finely divided as to be almost fibrous. The number originating from 

 the crown varies from 15 or less to 30 or more. Some of these roots take a 

 downward course from the crown, but many of them pass out at a slight angle 

 from the surface to a distance of 4 to 12 inches. The point from which they 

 start varies from a few millimeters to 2 inches below the surface of the ground, 

 and the point at which they turn downward varies from 2 to 8 inches. These 

 roots then penetrate to a maximum depth of over 3 feet. The diameter of 

 these roots at the starting point varies from 0.2 to 1 mm. Throughout the 

 surface soil they are branched and rebranched to the third and fourth orders, 

 and each group of branches varies from 1 mm. to 3 inches in length. Below 



6 inches these main roots, after starting downward, taper until they are not 

 more than 0.2 mm. in diameter. They are more or less branched, the branches 

 being almost hairlike in diameter and 3 to 10 mm. long. In color, the roots 

 vary from a very light brown to a pure white. They are extremely brittle and 

 easily broken. Five plants were examined (fig, 19). 



Asclepias verticillata pumila. — This dwarf milkweed is generally distributed 

 throughout the hard lands of the plains. It forms extensive midsummer clans. 

 The stems vary from 3 to 5 inches in height and are connected by an extensive 

 underground system which varies in diameter from 1 to 2 mm. These con- 



