230 THE PLANT WORLD 



Opuntia bigelovii, which is so densely clothed with short 

 silvery spines that a pencil point cannot be thrust against the 

 stem without pushing several aside, is abundant around the 

 mouth of Pima Canyon and shows an extremely wide range 

 over the deserts of the Southwest from Death Valley across 

 to the mesas of Arizona and southward along the shores of 

 the Gulf; it also propagates itself by means of detached joints, 

 and an entire colony of these plants may be seen that have 

 come from one older central individual. 



Among the more slender forms of this type, the less 

 common Opuntia tetracantha has the size of the chollas, 

 but its thinner reddish stems easily distinguish it from the 

 tasajo, while a second form, O. arbiiscuhi makes a low dense 

 clump not above a yard in height, the third species with 

 thin stems being O. leptocaulis, in which the diameter is 

 scarcely that of a lead pencil, the entire plant being much 

 smaller than the last named species and quite inconspicuous. 



A critical examination of the slender opuntias by Dr. 

 J. N. Rose during numerous field trips in 1908, resulted in 

 the discovery of a species hitherto unrecognized, which be- 

 cause of the profuseness with which it propagates by de- 

 tached joints has been named Opuntia vivipara. Practically 

 all of the space underneath the spreading branches is occupied 

 by plantlets arising from rooting joints, broken off by the 

 wind. The roots of this species display some unusual storage 

 structures. 



Several species of birds make their nests in the branches 

 of the cylindrical opuntias, where they are secure from 

 hawks and marauding animals; and many rodents of the 

 desert drag the detached joints about their burrows, making 

 an effectual barricade against the coyote and the fox. 



