A STUDY IN PLANT ADAPTATION. 487 



eeeds 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 choUa 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 choUa have a dif- 

 ferent structure from that found in the deeper roots and in the roots of 

 utost 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 choUa to absorb large quanti- 

 ties 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 par- 

 ticularly 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 



