MacDougal — The Water-balance of Desert Plants . 89 
after its weight had decreased nearly 50 per cent. This plant was exposed 
in the open. 
An Iberville a tuber lost at the rate of 08 g. in May, 1910, and at the 
rate of buto-i2g. daily in May, 1911, after the weight had decreased 15 per 
cent. Green stems were present, and the conditions fairly equivalent in the 
two periods. 
Ibervillea No. 4 lost at the rate of 08 g. daily in 15 days ending 
May 2 6, 1910, during which time new stems were starting, and at the rate 
of o*i6g. daily under equivalent conditions in 1911, after the weight had 
decreased 15 per cent. 
Ibervillea No. 5 lost at the rate of nearly 0-7 g. daily in 15 days ending 
May 26, 1911, under equivalent conditions, after the weight had decreased 
22 per cent. 
The green stems were especially vigorous in No. 5, and to their activity 
must be attributed the excessive total and high rate shown in May, 1911. 
An inspection of these data shows that a total loss of 15 per cent, in weight 
was followed by a decrease of 85 per cent, in the rate, a loss of 8 per cent, in 
weight by a decrease of 80 per cent, in the rate, a loss of 22 per cent, in 
weight by a decrease of 56 per cent, in the rate, in tubers of Ibervillea . The 
significance of these measurements is partly obscured by the activity of newly 
developing green stems, but it is evident that the major variation in the 
transpiratory loss with progressive depletion of the water-balance (which in 
turgid individuals would be 60-80 per cent, of the total weight) is not to be 
attributed to climatic variation, or the activities of stems formed, although 
the influence of the last-named feature is obvious. 
The evidence furnished by the individuals of Echinocactus free from 
the influence of factors which would prevent a fair comparison affords 
some further illustration of the matter. 
Echinocactus No. 1 lost water at the rate of 109 g. daily in March, 
1908, and at the rate of 3-5 g. daily in March, 1909, a decrease of 50-66 
per cent, in rate with a lessening of the water-balance of about 7-7 per 
cent. The maximum rate in May, 1908, was 29 g. daily, in June, 1909, 
15 6 g. daily, a decrease of nearly 47 per cent, in rate with a loss of 7-2 
per cent, in weight. 
Echinocactus No. 4, which had already been exposed in the open for 
eight months, lost at the rate of nearly 0-45 g. daily in 182 days ending 
May 21, 1909. The loss during the 209 days ending May 13, 1910, was 
at the rate of about 0*24 g. daily, a decrease of nearly 50 per cent, in 
rate after a total decrease of about 7 per cent, of the earlier weight. 
Echinocactus No. 6 lost at the rate of 6 g. daily for 155 days ending 
May 12, 1909. In 177 days ending May 13, 1910, the rate was 3 g. daily, 
a decrease of 30 per cent., although the weight had fallen but 47 per cent. 
The rate was but 2-3 g. in 157 days ending April 10, 1911, 
