the Intrinsic Transpiring Power of the Plant. 355 
The results of the experiment described above, which is one of about 
twenty similar ones, do not agree with the conclusions of Shreve ( 12 ). 
This writer remarks (p. 14) : ‘ It was found that while a marked increase 
in wind velocity was always accompanied by an increase in the actual trans- 
piration rate, this increase did not appear in the relative transpiration, thus 
showing that the plant was affected in the same manner as the atmometer.’ 
The experiments in question were carried out with potted plants in the 
open air, with conditions no doubt varying continuously and irregularly, 
as the graphs of evaporation show. Unfortunately no comparison was 
made, in the tables and graphs given, of the transpiration and evapora- 
tion with different wind velocities. 
* 
Livingston (6), in comparing the relative transpiration method with 
the direct method in which hygrometric paper is used, states (p. 18): 
‘ That the relative transpiration graphs exhibit marked accelerations and 
retardations that are only slightly, or not at all, indicated by the graph 
from the direct method seems to be explained by the supposition that the 
atmometers are much more sensitive to changes in the surrounding con- 
ditions than are the plants.’ The experiment was carried out in the open 
air and it seems not unlikely that some of the irregularities in the relative 
transpiration graph are to be explained by the different response of plant 
and atmometer to air movement. The graphs from the direct method 
would fail to exhibit these irregularities owing to the fact that the por- 
tion of the leaf under observation is protected from air movement by 
the hygrometric paper used. 
In their capacity to influence the rate of water loss from a leaf 
and an atmometer, relative humidity and temperature stand in contrast 
to air movement, for there is no a priori reason why the first two factors 
should not affect equally the rate of transpiration and the rate of evaporation 
from an atmometer. Air movements affect the diffusion gradient, on which 
the rate of water loss depends, by altering the distance between the evapo- 
rating surface and the point of minimum water concentration in the manner 
which has been indicated above, and this alteration is not the same in plant 
and atmometer. On the other hand, changes of relative humidity and 
temperature affect the diffusion gradient by altering the (water) concentra- 
tion difference or diffusion potential, which will influence plant and 
atmometer alike, irrespective of structure. Thus, subject to differences 
of conductivity of heat and of specific heats, the response of the plant 
to temperature and relative humidity changes should be the same as 
the response of the atmometer. 
Experiments have been carried out to ascertain the influence of 
changes of temperature and relative humidity upon plant and atmometer. 
