BIOLOGICAL RELATIONS OF CERTAIN CACTI ^ 



W. A. CANNON 



Especial interest is attached to the study of the cactus family 

 because it is peculiarly well adapted by habit and by structure 

 to withstand the trying conditions of the desert. The greatest 

 development of the group occurs in the arid portions of tropical 

 and subtropical America, but the conception which this statement 

 is likely to give that the cacti thrive best with a modicum of water 

 and live in localities that are too severe for all other desert plants 

 to endure, is erroneous. The cacti like other plants of the desert 

 are most vigorous when the water supply is adequate, and it is 

 by no means certain that such a form as the giant cactus {Cereus 

 giganteus) or the barrel cactus (Echinocactus wislizeni), as well 

 as the larger Opuntias, do not require a larger amount of water 

 than many of the large plants of other families. 



However, it is also likely that no desert plants can live and 

 perpetuate their kind under more arid conditions than some of 

 the cacti. The general means by whicli tlicy accomplish this are 

 too well known to require repetition hut ccrtaiti adaptations 



to desert conditions, not so well known ])crlia])s, may be pointed 

 out. The most important factor in the life relations of the des- 

 ert plants is unquestionably the available water supply, and the 

 most striking adaptations accordingly are associated with the 

 absorption, the storage, or the conservation of water. This rela- 

 tion to the water supply, either in apparent independence of it 

 or in intimate association with it, is met at each stage of devel- 

 opment. For instance, the seeds of the giant cactus will ger- 

 minate in summer while lying on the top of air-dry sand and 

 without previous wetting. Seedlings of Opuntia versicolor are 

 provided with water-storage organs (Fig. 1) although such are 

 absent in the adult plants. The reaction of the mature plants 

 to a variable water supply is also noteworthy. Specimens of 



' Papers from the Desert Botanical Laboratory of the Carnegie Institution, 

 No. 11. 



27 



