Page:Popular Science Monthly Volume 61.djvu/494

488 surface is chiefly a result of the elimination of the leaves as foliage and the contraction of the branches into thick, short stems as shown in Fig. 3. During the growing season the cholla exposes to the atmosphere less than one fiftieth of the surface which is exposed by the maple of equal weight here in the east.

Aside from this remarkable diminution of surface, the thick epidermis of the plant almost precludes transpiration at times when the water in the storage tissues begins to run low. The following illustrates how well the plant is adapted for conserving the moisture previously stored in its thick stems. I have cut a branch from the tree in the spring prior to the season of blooming, at a time when the storage tissue was well filled with moisture. I have placed these branches in a perfectly dry room out of contact with moisture. I have seen them continue in growth, and ultimately blossom. I have placed branches of the previous season's growth in open boxes without soil and without access to moisture early in March, and in September have found the branches still succulent and in condition to root and grow when placed in the soil.

Several years ago I removed a large specimen, having a trunk diameter of eleven inches and a height of ten to twelve feet, from the open mesa where it was growing to my garden. The tree was moved in late May when in full bloom. Although the month was hot and dry, the roots were closely pruned and the top left unpruned. The flowers did not wilt as a result of this severe treatment and a full crop of fruit ripened in the fall. This tree, which is illustrated in Fig. 4, suffered no apparent harm in its removal, although probably fifty to one hundred years old.

As a summary it may well be said that the cholla is admirably adapted for absorbing water rapidly, storing large quantities of it, and even when exposed to a very dry atmosphere for a long time retaining it with wonderful tenacity. Provision is also made for undue loss of moisture at times of injury to the epidermis, and the consequent direct exposure of the storage tissue to the dry air. At such times a mucilaginous substance contained in the cells appears on the injured surface, quickly rendering it impervious to moisture.