MacDougal. — The Water-balance of Desert Plants . 79 
rate fell to 8 g. in January, 5*5 g. in February, and rose to 67 g. daily when 
removed to the open in May, June, and July. The rate fell to 6 g. daily in 
October, 1910, and to 3-8 g. in the period of 89 days ending February 6, 
1911. The loss during a part of February, 1910, was one part in over 8,700, 
while in the corresponding period of a year later it was but one part in over 
9,300. The last calculation was made for a longer period, and the actual 
minimum may have been much lower. 
The total weight of the plant was reduced from 45*325 kg. to 34*810 kg., 
a total loss of 10535 kg., or 23-3 per cent, of the original weight. The 
chemical analysis made in April, after some further loss had been undergone, 
showed the following : — 
Total solids in juice per 100 c.c. . . . 10*44 g. 
Ash content of sap per 100 c.c. . . . 1*83 g. 
Three determinations of the osmotic activity by the freezing-point 
method gave 10*15, 9-64, and 9*72 atmospheres, the average being 9*84 
atmospheres at 25 0 C. 
Three analyses of growing plants previously reported 1 show total 
solids in the juice ranging from 3*4 g. to 5*9 g. with an ash content of 1 to 
1*7 g. per 100 c.c. of sap. 
The desiccation of this plant, which had progressed without any reple- 
tion of the water-balance for over 1 6 months, may be taken as representing 
about the limit of endurance of the species without water, both as to time 
and amount of loss. It is of interest to note also that determinations 
of the osmotic activity of the sap of individuals rooted in moist soil by the 
freezing-point method gave pressures of 6*78 atmospheres, although some 
as low as about 4 atmospheres by plasmolytic methods are on record. 2 It 
is to be seen from this that the range of variation with regard to this 
condition is as 1 to 2. 
The desiccation of Carnegiea was not found to cause any noticeable 
reduction in length, a fact that may be readily understood when the heavy 
woody cylinder is seen to be well formed to within a few centimetres of the 
upper end of the trunk. The shrinkage in the cortex would result in some 
decrease in diameter, but this would be accomplished by contraction of the 
folds without any great decrease of the outer surface. Turgid trunks con- 
sist of over 90 per cent, water, but their slender cylindrical form would operate 
to give them a relatively lower succulence than Echinocactus . Furthermore, 
the degree of succulence would be much lowered during desiccation by the 
maintenance of the surface area as suggested. Transpiratory loss seemed 
to be more or less closely correspondent to the succulence, and to be 
1 In Publication No. 141, Carnegie Institution of Washington, 1910, p. 47, 1911. 
2 See MacDougal and Cannon : Conditions of Parasitism in Plants. Pub. 129, Carnegie Institu- 
tion of Washington, 1910, p. 25. 
