310 BOTANICAL GAZETTE [APRIL 
postulated change would not necessarily result in a decrease in the 
actual rate of water loss, thus forming a maximum point in the 
graph of absolute transpiration, but might merely retard the increase 
in this rate and thus give that graph a lower slope as it approaches 
its later-occurring maximum. The graph of relative transpiration, 
however, should show a definite maximum at the point when this 
retarding change was applied, since this graph is entirely inde- 
pendent of the direct effects of variations in the evaporating power 
of the air. 
The maximum of absolute transpiration might occur at the same 
time as that of relative transpiration (when the postulated retard- 
ing influence may be considered as of greater magnitude than the 
accelerating influence of the still increasing evaporating power of 
the air), or it might occur later (when the acceleration due to the 
increasing evaporation rate may be considered as of greater 
influence than is the internal retardation). In fig. 8 of the publi- 
cation to which we are referring, two of these maxima in the graph 
of absolute transpiration (“Rta”) occur simultaneously with 
maxima in relative transpiration, while another occurs later than 
that in relative transpiration but earlier than the maximum in the 
graph of evaporation. The first two cases fulfil the former and the 
third the latter of the two suppositions made above. 
In seeking to examine this daily retardation somewhat more 
closely, it was found (Publ. 50 still) that the maximum in relative 
transpiration thus evidenced might occur at almost any hour before 
the evaporation maximum, but that its occurrence usually fell in 
the hours between 10:00 A.M. and 1:00 P.M. This maximum, 
furthermore, appeared to be related to temperature. ‘“‘As far as 
the limited data at hand can be trusted, the temperature of the 
surrounding air seems to be the controlling condition which governs 
this regulative response... . . There is some evidence that in- 
tensity of evaporation is the controlling factor, in some cases at 
least.””? 
From eleven cases on the graphs of the contribution just cited, 
we have calculated the amount of decrease in relative transpiration 
2 Livincston, B. E., The relation of desert plants to soil moisture and to evapora- 
tion. Publication 50 of the Carnegie Institution, 1906, pp. 63-64. 
