62 THE ECOLOGICAL RELATIONS OF ROOTS. 



almost vertically downward, following a more or less irregular course through 

 the hard soil, branching as shown in figure 21 and reaching a maximum depth 

 of 35 inches. While the shallower roots were brownish in color and very- 

 tough, these deeper roots were glistening white and much more fragile (cf. 

 Preston, 1900). 



Fig. 21. — Opuntia camanchica. Fig. 22. — Top view of surface roots of Opuntia caman- 

 showing vertical anchorage chica, showing the wide lateral spread, 



roots. 



Opuntia fragilis. — ^This cactus has a distinctively shallow root system. On 

 all sides roots extended out from the base of the plant, ranging from 1 to 2 mm. 

 in diameter, some having a lateral spread of 16 inches. There were about 20 

 on the plant examined. Most of these took a course parallel with the 

 surface of the soil and lay within 3 inches of the surface, but a few took a 



