168 The Philippine Journal of Science 1922 
A test upon a coconut leaf was made by holding a dried slip 
in contact with the lower surface of the leaf by means of a small 
glass clip, and noting the number of seconds required for the 
color change from pale blue to pink. The leaf was shaded dur- 
ing the test by means of a cheese-cloth screen, and the tempera- 
ture of the air, which was assumed to be practically the»-same as 
that of the leaf, was recorded.’ . 
As has been shown by several studies, the length of the time- 
period for a given color change over the standard evaporating 
surface depends, for any slip of paper, upon the temperature 
alone, this time-period being inversely proportional to the max- 
imum vapor pressure of water corresponding to the given tem- 
perature.* 
An example may illustrate the way in which calculations are 
made. In one of the tests, for a certain slip of cobalt paper the 
time-period for the color change on the leaf was 77 seconds when 
the air temperature was 23.7°C., corresponding to a vapor 
pressure of water of 21.76 millimeters. Since the slip of cobalt 
paper had required 31.9 seconds for the change over the standard 
surface when the temperature was 27.4° C., corresponding to 
a vapor pressure of 27.10 millimeters, the time-period required 
for the change over the standard surface at the temperature of 
the leaf would have been 31.9 x 27.10 ~ 21.76, or 40 seconds. 
In this test the index of transpiring power, being the time re- 
quired for the change over the standard surface divided by the 
time required for the change over the leaf at the same tem- 
perature, was 40 divided by 77, or 0.52. In making the calcu- 
lations the table of vapor-pressure ratios given by Livingston 
and Shreve(10) was employed. 
Tests were made upon coconut plants growing in the open 
field on April 29, April 30, May 3, May 4, May 7, May 8, May 9, 
May 10, and May 11, 1918. The experiments were conducted 
at the College of Agriculture, of the University of the Philippines, 
at Los Bafios. The writer is indebted to Messrs. F. de Peralta 
and P. David for assistance in the experiments. Column 2 of 
Table 1 gives the average actual values derived from these tests, 
showing the indices of transpiring power for the various hours 
during the day and night. Since similar plants were used for 
all the tests and the environmental conditions did not vary 
*See Edith B. Shreve.(13) 
‘See Bakke,(1) Livingston and Shreve,(10) and Trelease and Living- 
ston. (14) 
