MacDougal.— The Water-balance of Desert Plants. 77 
1910. The rate rose to 14 g. daily during the period ending February 6, 
1911, and the external dimensions remained unchanged from the measure- 
ment of October 5, 1910. During this period of eighty-nine days the 
external measurements did not follow the transpiratory loss. 
The final weight, taken on March 20, 1911, was 25.540 kg., indicating 
a total loss of 455 g. in forty-two days at the rate of nearly 11 g. daily. The 
total loss in thirteen months amounted to 23-850 kg., or 48-3 per cent, of the 
total weight. The total solids in the sap amounted to an average of 3-7 g. per 
100 c.c., the ash being 1-4 g. per 100 c.c. The average of a large number of 
freezing-point determinations gave an osmotic activity of about 3 atmo- 
spheres at 25 0 C. These results show a different aspect from anything 
previously examined. The plant seemed alive as indicated by its osmotic 
pressure, but the soluble matter in the sap had' been reduced to a minimum. 
Some interesting relations appear when the amount of loss is contrasted 
with the degree of succulency. Turgid specimens of Echinoc actus contain 
90 to 95 per cent, of their total weight in water, and this plant may be estimated 
to have contained about 45 kg. of water at the beginning of the experiment. 
The cylindrical body, with its fluted surface, offers about twice the trans- 
piratory area of a plain cylinder of the same measurement, which would 
amount to about 1 5,000 sq. cm. The degree of succulence would, therefore, 
be about 3 or 3 g. of water present to each sq. cm. of surface. This is to be 
compared with 10 g. in leaves of Siiaeda as determined by Miss Delf. 1 In 
this condition the plant lost in the open one part in 300 on the average 
during a period when daily temperature ranged from 32° to 90° F., and in the 
full blaze of the desert sunlight. 
The total loss in weight up to May 12, 1910, was 12-490 kg., which may 
be taken as being principally water, the total amount remaining now being 
about 32-5 litres. The surface had shrunken to 14,000 sq. cm., but the 
succulence had also decreased to about 2-3. These conditions, together with 
the higher relative humidity prevalent in July, were seen to result in a 
lessened rate of loss during the summer. At the end of the dry after- 
summer, the surface showed a further estimated decrease to about 3,000 sq. 
cm., and as but small loss had ensued since the last observation, the total 
now being 12-485, the remaining water may be estimated to have been 
about 32,000 litres, which indicates a succulency of 2-4 g. per 100 sq. cm. of 
surface, which is slightly greater than that shown in midsummer, although 
the rate of loss was much lower. Other factors may be taken as contributing 
to this result. The approximated position of the folds and the cooler night 
temperatures would both tend to lessen the rate of total loss. Next it is to 
be seen that as soon as the transpiratory loss was not taken up by the 
changes in external dimensions, the rate increased even under the retarding 
1 See Transpiration and Behaviour of Stomata in Halophytes, Ann. of Bot., vol. xxv, 1911, 
pp. 480-505. 
