224 



TliE PLANT WORLD 



The rigors of this arid season put the native animals 

 to the utmost test of endurance and many of the rodents have 

 recourse to the supphes of water in the storage stems of 

 the opuntias, eating away whole joints for the mixture of 

 mucilage, water and acid which they contain. 



The gardens of older Tucson contain many large speci- 

 mens of two opuntias native to the tropics or southern 

 Mexico, Opuntia jiciis-iiidica and "tuna castilla," both 



Fig. 6. Opuntia chlorotica, a spineless prickly pear of Southern Arizona. 



devoid of spines and cultivated for the sweetish fruits and 

 succulent tender stems. 



The cylindrical opuntias Include many forms with a 

 central stem and well developed system of branches which 

 give them the form and Imposing appearance of trees. These 

 begin activity early In April, two of the earlier ones being 



