488 



POPULAR SCIENCE MONTHLY. 



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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 di- 

 minution of surface, the thick epi- 

 dermis of the plant almost precludes 

 transpiration at times when the 

 water in the storage tissues begins 

 to run low. The following illus- 

 trates how well the plant is adapted 

 for conserving the moisture pre- 

 viously 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 stor- 

 age tissue was well filled with mois- 

 ture. I have placed these branches 

 in a perfectly dry room out of con- 

 tact 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 hun- 

 dred 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 mucilagin- 

 ous substance contained in the cells appears on the injured surface, 

 quickly rendering it impervious to moisture. 



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Fig. 3. The Short, Thick, Succulent 

 Stems of the Cholla bearing Clusters of 

 Fruit and Flowers. 



