90 MacDougal . — The Water-balance of Desert Plants. 
a decrease of 23 per cent, in the rate although the weight had fallen but 
3-5 per cent. 
Echinocactus No. 13 lost at the rate of i5o*5g. daily in 83 days 
ending May 12, 1910, and after a decrease of 28-6 per cent, of the re- 
maining weight, the rate was less than 11 g. daily, a decrease of 92*6 
per cent. 
Five possible causes which might have influenced the rate of trans- 
piration of a desiccating succulent present themselves. These are as follows : 
1st, the increased concentration of the cell-sap, which was of such degree 
in the experiments as to increase osmotic pressures from 4 or 5 to 10 or 
12 atmospheres, might retard evaporation from the cell-membranes ; 2nd, 
a diminution of the degree of succulence, or proportion of water per unit 
area of surface present might lessen evaporation ; 3rd, desiccation may 
result in alterations in the character of the outer membranes, or of any of 
the transpiring walls of the plant ; 4th, desiccation may stimulate the 
formation of new tissues or the alteration of existing cells in such manner 
as to close openings through which water vapour might pass ; and 5th, the 
positions of the surfaces might be shifted in such manner as to vary the 
exposure and lessen transpiration. 
Livingston has recently pointed out that a concentration of the sap, 
even if carried to a point where an osmotic pressure of 100 atmospheres 
was exhibited, would not give a retardation of more than 10 per cent, from 
the rate afforded by a pure water surface. 1 It is evident, therefore, that 
this factor is negligible in the present discussion, as the increases found 
were not more than 5 or 6 atmospheres. 
The records of the observations are not sufficiently complete to permit 
any accurate integration of the influence of succulence upon the rate of 
transpiration. The course of events in the behaviour of Echinocactus 
No. 13 shows a fair correspondence between the two in the course of rapid 
desiccation in the open air. The rate of loss diminishes so much more 
rapidly than the estimated degree of succulence in slowly d&iccating 
individuals that it is impossible to escape the conclusion that other agencies 
are operative. Thus, Echinocactus No. 1, weighing over 42 kg. in March, 
1908, may be safely estimated to have contained at least 92 per cent, or 
39 kg. of water, and by comparison with No. 13 would have a relative 
succulence of 3 and a surface of 13,000 sq. cm. The rate of transpiration 
was 10 g. daily. A year later the degree of succulence after a loss of 3 kg. 
may be estimated at 2*76, with no allowance for diminished external area. 
The rate of loss, however, was 5 g. daily, a decrease of 50 per cent, although 
the succulence had fallen but 8 per cent. If an allowance were made for 
the shrinkage of external area, the disproportion would be greater, as the 
1 Livingston, B. E. : The Relation of Osmotic Pressure in Plants of Arid Habitats. Plant 
World, vol. xiv, 1911, p. 153. 
