Mac Doug al . — The Water-balance of Desert Plants. 75 
the laboratory. The high temperatures and intense sunlight soon killed 
it, however. (See PI. VII.) 
Echinocactus No. 6 was taken from the soil November 7, 1908, and 
after being freed from roots was mounted on a stand in the laboratory. 
The initial net weight was 28-573 kg., and the rate of loss during the first 
month was 13 g. daily, which was about one part in 2,200 of the total 
weight. The rate during the winter fell to o-6g. daily, or one part in 5,000, 
and rose in the hot fore-summer to iog. daily. By October, 1909, the rate 
had fallen to 8 g. daily, and to 2-5 g. daily in November, which was one-fifth 
of the rate of the previous year. The rate in January was 1*7 g. daily, or 
one part in 16,000. Observations now showed 2-2 g. in March, 4 *6 g. in 
April, 6 g. in May, 7-2 g. in June, 7-7 g. in July, 5-8 g. in August, 5 g. in 
November, and 2 g. daily in the period extending into February, 1911, a rate 
of one part in over 12,000, which was greater than that of the previous year, 
but was still less than the rate of loss during the first year of observation. 
The rate increased to 2-6 g. daily during February as the room was heated 
daily during that time. The rate of loss during the first seventeen days 
of March was nearly 4g. daily, and 2*3 g. daily during the twenty-four 
days ending April 10. The entire loss during twenty-nine months was 
3-7 13 kg., or nearly 13 per cent, of the original weight. No noticeable growth 
had taken place, but sixteen greenish-yellow fruits were retained. It is to be 
seen that the changes in the form of this plant would be such as to maintain 
fairly constant transpiratory conditions, and the variations from year to 
year are to be ascribed to other causes than variations in temperature, air 
currents, &c. 
Echinocactus No. 7 was taken from the soil November 7, 1908, freed 
from its roots, and set on a metal support in the laboratory. The net weight 
was 35-818 kg., and the rate of loss during the first month was 18 g. daily, or 
about one part in 2,000, which was slightly more than that of No. 6. The 
rate during the winter was 10-5 g. daily, or one part in about 3,500, which 
was higher than in No. 6. The rate ran very irregularly, falling to 8 g. daily 
in June, 9 g. in August, and decreasing to 4 g. in November, which, like that 
of No. 6, was much less than in the previous year. The winter rate did not 
fall below this, however, and amounted to one part in over 8,000, which was 
double the rate of No. 6. The loss rose to 9-8 g. daily in June, fell to 6 g. 
in August to October, to 4 g. daily in October, while the midwinter rate, 
.1910-11, was 2-i g. daily, or one part in 15,000, less than that of No. 6 and 
slightly less than its own rate of the previous year. The rate from February 3 
to February 28 was i-6g. daily, a notable decrease and less than that of 
No. 6 during the same period. The erratic record from February to April 
was found to be due to errors caused by a mouse’s nest built among the 
arms of the balance. During the next seventeen days the rate rose to 2 g. 
daily, and to 3-3 g. daily during the twenty-four days ending April 10, 1911. 
