232 Knight . — The Interrelations of Stomatal Aperture , 
only the periods ending at 1.20 p.m. and 2.20 p.m. showing a loss to the 
plant of 10 mg. in each case. 
The plant, therefore, did not suffer from lack of water as in the case of 
Experiment 12, and consequently there was no incipient drying to check the 
rate of transpiration. The result was that relative transpiration continued 
to increase for some time after the maximum evaporating power of the air 
had been reached. 
The whole series of experiments shows the same tendency as that indicated 
above . When the amount of the water lost by transpiration was markedly 
less than the amount absorbed, the maximum of relative transpiration 
regularly fell before that of evaporation. When the plant absorbed more 
water than it transpired there appeared to be no check upon the rate of 
transpiration, and relative transpiration showed no maximum value early in 
the day, and sometimes, as in Experiment 11 above, the maximum was not 
reached until the stomata were nearly closed. 
No case has been found in which actual transpiration has been equal to 
absorption throughout the experiment, but in several cases transpiration has 
been greater than absorption during some periods and less than absorption 
during others. This state of affairs tends to make the graph of relative 
transpiration rather irregular with no definite single maximum. In these 
experiments it is not possible to correlate each change of water-content of 
the plant with changes in relative transpiration. It is possible that this is 
due in part to the inevitable defects of the method of dividing the experi- 
ment into half-hour periods for transpiration determinations, coupled with 
the possibility of a lag in the influence of the quantity of water absorbed by 
the stem at one end of the shoot on the rate of transpiration from the leaves 
some distance away. 
It should be noted here that in Experiment 12 quoted above, the actual 
transpiration rate reached a maximum simultaneously with the relative tran- 
spiration rate, although this does not occur regularly when the relative tran- 
spiration maximum falls before that of evaporation. The theory of incipient 
drying is not sufficient by itself to account for the fall of the actual transpira- 
tion rate, although it explains the decrease of relative transpiration. If tran- 
spiration, absorption, and the water-content of the leaf are related in the 
manner which Livingston’s theory suggests, then, other factors being 
constant, incipient drying will tend to check an increasing transpiration rate 
(and, conversely, an increase in the water-content of the leaf will tend to 
check a decreasing transpiration rate) and to bring it to a constant value in 
equilibrium w : th the rate of absorption. But transpiration will show no 
alternation of maxima and minima without the intervention of some factor 
other than incipient drying, e.g. stomatal closure or decrease in the evaporat- 
ing power of the air. Shreve appears to have overlooked this relationship 
in the explanation offered of the two maxima of transpiration (21, pp. 48 
