A STUDY IN PLANT ADAPTATION. 



491 



which became attached to them in their effort to get the fruit. The 

 spine arrangement of these end branches or burs is such that when 

 they finally become detached from the animals transporting them and 

 fall to the ground, the lower end comes in contact with the soil. As the 

 roots start from tins end of the branch the necessity for this provision 

 is very evident. It results from the spines being very short or wanting 

 on the lower end of the short, thick branches. 



The special adaptation of the fruit to aid in vegetal dissemination is 

 confined, so far as I am aware, to a few species of the Opuntia and 

 reaches its highest development in the plant that I have described above. 



Fig. 6. A Geotjp of Chollas, showing the large Clusters of Pendulous, Spineless 

 Fruit. Photographed by W. T. Swingle. 



The fruit of the cholla is probably changing from its original seed- 

 bearing condition to a condition of sterility. The abundant clusters of 

 fruit hang from the plant within easy reach of cattle and it is interest- 

 ing to note that since the advent of stock into the arid southwest the 

 cholla has become more widely distributed and more abundant than 

 ever before. It is as well equipped by nature to care for itself and per- 

 petuate itself on the hot, dry sands of the desert, as is the New England 

 elm of the humid east which bursts into foliage under April showers. 



