THE PLAINS ASSOCIATION. 



61 



Opuntia camanchica. — This Opuntia enjoys a very wide distribution in the 

 plains grassland. Its water requirement seems to be somewhat less than that 

 of Bouteloua. It is favored in its competition with the grasses by grazing, and 

 its development in great abundance is frequently indicative of overgrazing. 



A single, carefully selected plant of average size was examined. As is 

 characteristic of the cacti in general, the root system consists of two distinct 

 parts — a few vertically descending anchorage and deep absorptive roots, and 









-J- — 









^ 









V 











^ 



V 







Fig. 20. — Underground parts of Asclepias verticillata pumila. 



a much more extensive and shallow surface absorbing system. An examina- 

 tion of figure 22 shows 23 roots which run off in the surface soil, usually at 

 a depth of about an inch and seldom deeper than 3 inches, to distances 

 varying from 6 inches to 6 feet. The two largest roots were 5 mm. in diameter 

 and tapered very slowly in spite of the repeated branching. They were 

 branched repeatedly from their origin at the base of the plant to their 

 extremity with both large and small branches, which ramified in all directions 

 and thus furnished an enormous absorbing surface. The ultimate root end- 

 ings, whether of the shorter or larger branches, consisted of much-branched 

 and very delicate brushlike termini, to which the soil clung with great tenacity. 

 The smaller branches, except for their lesser extent, are sinailar to those already 

 described. 



The deep anchorage and absorbing system of this plant consisted of 4 main 

 roots, which at the outset were 2.5 to 3 mm. in diameter. These extended 



