36 THE PLANT WORLD. 



specially developed tissues of the stem, root, branches or 

 leaves. While this feature is exhibited most abundantly by 

 plants of arid regions in which the rainfall comes within a 

 brief season, yet it is also found in a goodly number In moist 

 tropical jungles, and caution in designating it as a direct 

 adaptation seems desirable. Before much progress may be 

 made in the analysis of the storage function it will be neces- 

 sary to come at the chemical mechanisms by which it is made 

 possible, together with an analysis of the environmental 

 forces which incite or inhibit it. Twelve or thirteen hundred 

 species of cacti exhibit this capacity in varying degrees, and 

 it is this chiefly which givesthem their possibilites as a valu- 

 able forage plant. Our optimism as to the possible benefits 

 must not blind us to the fact that nine-tenths of the body of a 

 cactus is water and that no more than a reasonable fraction 

 of the amount present in the surface layer of soil two or 

 three feet in depth can be taken up and made available for 

 the construction of stems, or accumulated in storage organs. 

 It is, however, one of the most promising investigations yet 

 undertaken looking toward the possible utilization of native 

 desert plants. 



Primarily but indirectly the comparative struggle for 

 existence in the desert is one for water, not among plants so 

 keenly as between animals and plants. If it were possible 

 to exclude certain birds and mammals from the domain of 

 the Desert Laboratory, the Included square mile would soon 

 become a dense forest of sahuaro {Cereiis giganteus) . In 

 addition to the enormous consumption of seeds, almost all 

 of the seedlings, which have a swollen hypocotyl, are appro- 

 priated by animals since these present about the only avail- 

 able supply of water in the dry after-summer during which 

 they come Into activity. So keen is the quest for this material 

 that the few dozens that may be grown in the experimental 

 greenhouses of the Desert Laboratory are Inevitably de- 

 stroyed unless guarded. The actual balance then is not be- 

 tween the sahuaro and other plants, but between this great 



