MacDougaL — The Water-balance of Desert Plants . 91 
actual decrease in succulence was probably not more than 5 per cent. 
Data from Echinocactns No. 6 for longer periods are equally interesting. 
This plant contained about 27 kg. of water December 8, 1908, and with a suc- 
culence of 3 would offer a surface of 9,000 sq. cm. The rate of loss for the 
155 days ending May 12, 1909, was 6 g. daily. November 18, 1909, 
a further loss, 2-335 kg., had ensued and the succulence was then 2-7 daily. 
In the 178 days to May 13, 1910, the rate of loss was but 2-8 g. daily. 
The succulence had decreased 10 per cent, and the transpiration rate 
53 per cent. The annual growth and consequent accelerated loss in 
Ibervillea makes the data from these plants ineligible in the present 
connexion. 
The approximation of the ridges or folds in Echinocactns would operate 
to diminish the angle of exposure to the sun and the consequent transpira- 
tory loss. A detailed examination of Echinocactus has not been made, but 
the corky outer layer of the tubers of Ibervillea is being added to at intervals, 
and changes of this character would lessen the water loss. 
The total losses from various plants in which the course of desiccation 
was not disturbed is given below : — 
Name. 
Original 
weight . 
Total 
loss. 
Propor- 
tion. 
Period. 
Retnarks. 
Echinocactus. 
„ No. 1. 
4 2 -743 kg. 
4-583 kg. 
n% 
15 mo. 
In shaded room. Died. 
„ No. 2. 
5-I36 kg. 
1-724 kg. 
33% 
18 mo. 
Shade and open air. 
Living. 
„ No. 4. 
1-192 kg. 
0.643 kg. 
54% 
36 mo. 
Died. 
„ No. 6. 
28-573 kg. 
3-713 kg. 
13% 
30 mo. 
Shade. Living. 
„ No. 7. 
35-818 kg. 
4-248 kg. 
n% 
30 mo. 
Shade. Living. 
„ No. 13. 
49-390 kg. 
’23-850 kg. 
48% 
13 mo. 
Open. Living. 
Carnegiea. 
» No - r - 
40-000 kg. 
11-250 kg. 
00 
12 mo. 
Open air and shade. 
Living. 
„ No. 1 a. 
45-325 kg. 
iQ-535 kg. 
*3%, 
16 mo. 
Open air. Living, 
20 seedlings. 
27-244 g. 
1 7*745 g- 
547% 
102 da. 
Shade. 4 dead. 
Brodiaea. 
36 resting 
corms. 
Ibervillea . 
46-700 g. 
18-425 g. 
39-5% 
96 da. 
Shade. Young corms 
formed. 
3 
p 
p 
530- g- 
143- g- 
27% 
19 mo. 
Shade. Sending up 
shoots annually. 
„ No. 4. 
I# 45 2 kg. 
164- g. 
1T '5% 
14 mo. 
Shade. Annual shoots. 
„ No. 5. 
1-006 kg. 
278- g. 
27% 
14 mo. 
Shade. Annual shoots. 
Tumamoca. 
Tuber 
125* g- 
34’4 g- 
J 7*5% 
4 mo. 
Shade. Died. 
No special attention was given to the matter of absorption of water or 
water vapour by the plants which were used in the experimental work. The 
results of the weighings, however, disclose the fact that small individuals of 
Echinocactns made a net increase in weight when placed in a dark room 
with a relative humidity of 80-90 per cent., after removal from a room with 
a humidity of 15-30 per cent. Other specimens made similar gains when 
