82 MacDougal. — The Water-balance of Desert Plants . 
The total loss from this plant in 19 months was 143 g. or 27 per cent, of 
the total weight. This included the material used in the construction of stems 
and leaves in the summer of 1910. In addition to the increased transpira- 
tion from these stems, some marked loss would be attributable to respiration 
and to the falling away of dead stems and branches. (See upper right-hand 
figure in PL X.) 
Iberville a Nos. 4 and 5 were received as freshly collected tubers from 
Dr. J. N. Rose, who was in the field near Guaymas, Sonora, in March, 
1910. The tubers were cleaned and freed from roots, then placed on suit- 
able supports in a shaded laboratory. The weight of No. 4 was 1-452 kg. 
on March 17, 1910. The loss during the first five days was 25-6 g. daily, 
which was about one part in 520. Much of this accelerated loss was due to 
the transpiration from cut surfaces and bruised tissues. With the closure of 
such places the rate fell to 1*3 g. daily in April, and 0-85 g. in May, although 
the rising temperature would have tended to increase the rate, which 
continued to fall until it was but 0-36 g. in June, which was one part in 
nearly 3,700. The development of a number of branching stems caused the 
rate to rise to o-66 g. daily in July, which fell to 0-5 g. in the period ending 
in early October, when the stems were dead. A decrease continued which 
brought the rate to less than o-2 g. daily in late October, and to 0-04 g. 
daily in the period ending February 3, 1911, which was but one part in 
30,000. The room was warmed to i8°C. for a few hours daily during 
February, and the rate of No. 4 rose to o«3g. daily in March, but fell 
to 0-05 g. daily in the 20 days ending April 10, but rose to 0-35 g. in 
the 21 days ending May 1. 
May 2 6, 1911, the weight was 1-260 kg., indicating a daily loss of 
0-16 g. in May, which was less than that in April, although green stems 
were being formed. A similar decrease in the rate of loss was recorded for 
the corresponding period of 1910 as noted above. No explanation may be 
offered for the erratic behaviour of this plant, which was kept within a few 
centimetres of the other tubers used. 
The total loss in 14 months amounted to 192 g. or but 13 per cent, of the 
original weight, although many branches and small leaves were produced in 
the summer of 1910. (See left-hand figure in PL X.) 
Ibervillea No. 5 weighed i-oo6kg. on March 17, 1910. The rate 
of loss during the following days was but 3-6 g., which was probably less than 
that of No. 4, largely by reason of the small amount of damaged surface. 
The rate fell to 0-9 g. in April, and to 0-5 g. in early May. The rising 
temperatures failed to accelerate the loss, but the rate rose only when green 
stems were formed. These brought the loss to o-6 g. later in May, 0-9 g. in 
June, and to 1-5 g. with the fullest development of the leafy stems in July. 
The gradual death of the stems brought^ the rate down to 1 g. daily in the 
68 days ending October 3, 1910, 0-9 g. in November, and to 0-15 g. daily in 
