74 MacDougaL — The Water-balance of Desert Plants . 
November, 1910, to February 3, 1911, or one part in 2,500, the glass shade 
having been removed. Its weight was 3-855 kg. on February 28, 1911, and 
after a rain of 0-19 inch in the form of a slow drizzle throughout the night, 
the same weight was found. Estimating the total loss at the rate in the 
preceding period, it must have amounted to 40 kg. in twenty-five days, and 
this amount appears to have been balanced by the absorption on the part 
of the dried spines and other external parts. It may not be asserted that 
any of the above absorption was carried on by living tissue, nor does it 
appear that the water taken up might be of use to the plant. 
The chemical analysis of the sap in April, 1911, showed 9-512 g. solid 
matter per 100 c.c. of sap and 3-864 g. of ash, the average of over a dozen 
determinations giving an osmotic pressure of 2*2 atmospheres at 25° C. The 
total solid and ash were thus about five times the minimum shown by 
a turgid plant (see p. 76). The concentration of the sap was greater than 
that of any plant previously examined, yet it is notable that the osmotic 
activity was scarcely more than half that of any other specimen examined. 
This would seem to indicate that actual disintegration had begun, and that 
this individual might not have been able to recuperate if supplied with soil 
moisture. 1 The total loss was 2-075 kg. or 36-2 per cent, of the original 
weight in three years. 
Echinocactus No. 4 was taken from the soil, the root-system cut away, 
and placed on a base of loosely piled volcanic rock in the open on March 5, 
1908. The estimated weight was 1-192 kg., which was reduced to 795 g. on 
November 5, 1908. A slight gain in weight was noted, as in No. 3, when 
this plant was brought into the shaded laboratory. The rate of loss was 
0-5 g. daily in the 164 days ending May 21, 1909. This was equivalent to 
one part in 1,500. The rate was o-6 g. in August, then fell to 0-4 g. and to 
o-2 g. in November, and then during the midwinter period fell to 0-13 g., or 
one part in 5,300. A gain of 2 g. in weight ensued when the plant was 
placed in the humid dark room for seven days. The humidity was 62 per cent, 
with a temperature of 58° F. Upon being taken out the rate rose to 0-4 g. 
in March and o-6 g. in May, which was practically equivalent to the rate at 
the corresponding period of the previous year. A rate of 0-4 g. was main- 
tained from July to November, when it fell during the period of November 
to February too-n g., or one part in 5,000, which, with corrections for varia- 
tion in periods of observation, is not materially different from the rate of 
the year before. 
The room in which this plant was placed was heated during February 
so that the rate rose to 0-24 g. daily and the weight fell to 549 g. on 
February 28. It was now placed outside for four days, its final weight 
on March 5, 1911, being 549 g. It was then set on the parapet wall of 
1 See MacDougal and Cannon : The Conditions of Parasitism. Pub. 129, Carnegie Institution 
of Washington, p. 33, 1910. 
