74 THE WATER-BALANCE OF SUCCULENT PLANTS. 



the ash as 1 to 3. The total solids dissolved in the sap of a turgid Car- 

 negiea amounts to 3.4 parts in 100, of which 2.4 is organic material and 1 

 ash. In the desiccated plant the dissolved solids amount to 9.6 parts in 

 100, of which 6.8 are organic and 2.8 ash. The general concentration was 

 as 3.4 to 9.6, the concentration of organic material as 2.4 to 6.8, and of the 

 ash as 1 to 2.8. 



The extremely great individual variability with regard to the rate of loss 

 of water makes it impossible to institute any comparison between the two 

 massive cacti employed, except to say that the water-balance was depleted 

 in very much the same general way in both forms. It is to be readily seen, 

 however, that the rate of loss is very much greater in these green plants 

 with their chlorophyllose stems than in the indurated tubers of Ibervillea, 

 which is in effect in a resting condition except during the brief periods dur- 

 ing which the thin vines are being developed. 



The depletion of the water-balance in the cacti is accompanied by revers- 

 ible changes in form and size, which may change the volume and appear- 

 ance of the trunks or stems very markedly. The repletion of the water- 

 balance necessitates reversible changes in form and volume, which may 

 or may not be accompanied by irreversible additions, due to growth or 

 morphogenetic changes. Some measurements of the bodies of massive 

 cacti are seen to be influenced by insolation and by temperature. 



Practically all of the species examined would be capable of endurance 

 for an entire year in the open, although the water-supply were cut off. 

 Not all of the individuals might survive, but some would. Slight growth 

 of the trunk was seen in desiccated individuals of the bisnaga, but none in 

 the sahuaro. Many individuals may be encountered in the open which 

 show no indications of growth during the previous year, suggesting a lack 

 of moisture. It is notable that even very much desiccated cacti which do 

 not display growth of the succulent stems by reason of the depleted water- 

 supply may still send out roots. The capacity of root-formation seems to 

 be retained so long as the plant lives. During the second year of depriva- 

 tion of water the rate of loss is very much lessened, and if a large number of 

 individuals of Echinocadus and Carnegiea under the diversified conditions 

 offered by their habitats be taken into account it would seem justifiable to 

 assume that a second season of desiccation in the open might be endured 

 by these plants. Individuals in shaded rooms were in good condition at 

 the close of three seasons' deprivation of water. These limits apply 

 especially to the green bodies, or stems, of succulents, the superficial lay- 

 ers of which are chlorophyll-bearing-, and in which the major part of the 

 photosynthetic work is carried out. 



The short tuberous stems of Ibervillea upon which the observations were 

 made sustain an entirely different morphological and physiological rela- 

 tion to the life of the plant. These bodies represent the bases of stems 

 the apical portion of which is ephemeral . While the upper portions of the 



