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Page:Popular Science Monthly Volume 61.djvu/493

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487
A STUDY IN PLANT ADAPTATION.

ceeds 100 degrees F. during the hottest part of the day. Often for several consecutive months there is no precipitation whatever and much of the rain that does fall only penetrates the. soil to the depth of a few inches.

Not only is this cactus, as an individual plant, perfectly equipped by nature to withstand and thrive under this extremely hot and arid environment, but it is splendidly equipped for perpetuating itself by its successful distribution of offspring under conditions which enable them to succeed where on account of lack of moisture most plants would perish.

The roots of the cholla do not penetrate to great depths in the soil as one would at first thought suspect them to do. For the most part they spread out a few inches under the surface. It would be useless for this plant to send its roots to great depth into the soil, because only in rare instances is there any available moisture there. As most rains only penetrate the soil to the depth of a few inches the most moisture is found in the surface soil. These surface roots of the cholla have a different structure from that found in the deeper roots and in the roots of most plants. Their structure is splendidly adapted to enable them to take up water with great avidity when the soil is moist and to survive long periods of drought during which the surface soil is practically air dry.

It would be of no special value to the cholla to absorb large quantities of water when available, if there were no provision made by the plant for storing it, or if through transpiration it were readily given up to the surrounding atmosphere. This cactus is not only remarkably well equipped for storing water in large quantities, sufficient to carry it through months of continuous drought, but it is able to retain this water with wonderful tenacity, only giving it up to the hot and dry air a little each day and taking advantage of each rain to fill its storage tissue.

The structure of the young branches and stems of the cholla particularly adapts them for the storage of water in large quantities. At the height of the growing season or after a prolonged summer rain the stems of this cactus may contain as high as ninety-two per cent, of moisture. During a prolonged drought the percentage of moisture very perceptibly diminishes. The older stems and branches which give strength and support to the tree contain a much larger proportion of woody tissue and consequently serve to a less degree for the storage of water.

The ability of the plant to retain moisture results largely from the comparatively small surface exposed to the dry air and the remarkably thick epidermis and dense spine covering of the branches. The small