Page:Popular Science Monthly Volume 61.djvu/497

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491
A STUDY IN PLANT ADAPTATION.

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 this 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.

 

PSM V61 D497 Chollas with large clusters of spineless fruit.png

Fig. 6. A Group 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 interesting 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 perpetuate 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.