2 THE WATER-BALANCE OF SUCCULENT PLANTS. 



absorbent functions during the season of vegetative and reproductive 

 activity or at some other time of the year. A large number of the species 

 existing under such conditions have a capacity for absorption and conduc- 

 tion of water far in excess of the rate of loss from the transpiratory sur- 

 faces. Such forms accumulate a large balance which may be contained in 

 swollen roots; in stems, as in the cacti and euphorbias; in leaves, as in the 

 yuccas and agaves; or in all of these organs, as in the Crassulaceae . 



The habit of accumulating a large water-balance is not a property of 

 any morphological type, nor is it confined to any group of forms capable 

 of being phylogenetically related. Succulents are prominent constituents 

 of the floras of salt springs and of the beaches of saline lakes and seas, as 

 well as of arid areas in which the scanty rainfall comes within a restricted, 

 regularly recurring seasonal period. Some are also found in tropical rain- 

 forests and in humid situations in temperate zones. Few occur in high 

 latitudes in which the effect of low temperatures on dilute solutions would 

 tend to be injurious to tissues. 



The Sonoran desert, inclusive of southern Nevada, Arizona, and Sonora, 

 with the coastal belt to the southward, is especially rich in forms which 

 habitually carry a large water-balance, and a number of these are abun- 

 dant in the vicinity of the Desert Laboratory, offering opportunities for a 

 study which has been prosecuted with some diligence since 1906, attention 

 being chiefly directed to the great tree-cactus {Carnegiea gigantea), the 

 bisnaga iEchinocadus wislizeni) , and some common prickly pears ( Opuntia 

 blakeana and O. discatd) . 



It may be safely assumed that practically all perennials carry an appre- 

 ciable balance, as the flow of solutions from absorbing surfaces to the vari- 

 ous tissues is by no means direct or by way of conduits that allow a clearing 

 stream. This balance may be large in trees and other woody plants, but in 

 none of these forms does it play such an important part in the cyclic activity 

 of the plant or constitute such an important feature in survival and endur- 

 ance as in the succulents of the arid regions. The condition of this balance 

 may be the determining factor which may inhibit or promote seasonal 

 activity, including growth, and variations in the balance are accompanied 

 by reversible changes in form and size unknown in other types of plants. 



The long series of measurements recorded in the present paper have 

 been made for the purpose of determining the amount of the balance, its 

 variations, the factors influencing its volume, and the relation of the vari- 

 ous proportions of the balance to growth and the reversible changes to 

 which such plants are subject. Some attention has also been given to 

 analyses of the sap for the purpose of ascertaining the concentration of the 

 solutions held by the plant and the implied osmotic activity. 



The data derived by these methods have permitted some conclusions as 

 to the action of selective agencies and survival in desert plants, and also 

 have justified some speculation as to the origination of succulent forms of 



