MacDougal — The Water -balance of Desert Plants . 81 
the water of the larger member from which it originated. The repetition 
of this action might result in the survival of the species through several arid 
seasons by the seasonal development of a series of diminishing size 
(PL IX, D). 
IBERVILLEA SONORAE. 
The indurated tubers of this plant are irregularly globose or flattened, 
and lie on the surface of the ground. A network of small fibrous roots is 
formed during the summer rainy season and serves to take in a supply of 
solution which is added to the enormous balance already present. The 
growing points which are distributed irregularly over the surface of the 
tuber awaken at the same time, and produce vines which may reach a length 
of two to three metres, quickly maturing flowers and fruits (Pis. VIII and 
X). The rhythm of the plant is such that this activity ensues for many 
seasons after the plant has been separated from the substratum, as has 
already been described. 1 
A tuber of a plant which had been established at the Desert Labora- 
tory in 1906 was taken up October 22, 1909, and when cleaned and freed 
from roots and dead vines, weighed 530 g. The loss was 0*4 g. daily during 
the first 15 days, or one part in 132, o-id g. daily in the next 12 days, and 
in the 51 days ending January 8, 1910, the loss was but 0-08 g. daily or one 
part in 6,5 00. The rate fell to 0-04 g. daily in the latter part of January and 
February, which was but one part in nearly 13,000, but rose to o*n g. daily 
during the 55 days ending April 21, 1910. 
The rising temperature stimulated the formation of green stems 
in May, the rate of loss rising to 0-3 g. daily, or one part in 1,500. The 
rise continued until the rate was 0-08 g. daily in July, which was one part 
in 573> this being the greatest transpiratory activity exhibited at any time. 
A decrease now began which brought the rate to 0-05 g. in the period 
ending October 3, 1910, and to 0-3 g. in November, after the vines had died. 
The rate during the following 91 days was 0*09 g. daily, or one part in nearly 
4,500, which it may be seen is in excess of the rate during the corresponding 
period of the previous year. The room containing this plant was warmed 
in February so that the rate increased to o-i6g. daily, the weight being 
389 g. on the last day of the month. The rate during the first 20 days of 
March was less than o-i g. daily, decreasing slightly during the next 
21 days, but rising again to exactly o-i g. daily during the 20 days ending 
May 1, 1911, slightly less than the rate of loss during the corresponding 
period in 1910. 
May 26, 1911, the weight was 387 g., indicating a daily loss of o-i2g. 
during May, an increase commensurate with the higher temperatures 
prevailing. 
1 MacDougal, D. T. : Botanical Features of North American Deserts. Pub. No. 99, Carnegie 
Institution of Washington, 1908, p. 20. 
G 
