1912] LIVINGSTON & BROWN—TRANSPIRATION 311 
which was manifest at the time of the evaporation maximum. 
Thus, at its maximum, the relative transpiration ratio of Euphorbia 
(Publ. 50, Carnegie Inst., fig. 8) is 0.069, this ratio falling rapidly 
and becoming only 0.035 at the time when the highest evaporation 
rate for the day is reached. Thus, during the period indicated 
the retardation of relative transpiration has amounted to 50.8 
per cent. On the second day this retardation amounts to 33.3 per 
cent. The average percentage decrease for the eleven cases is _ 
48.6 per cent, there being little variation in the terms of the series 
(33.3 is the smallest, the next higher is 41.8, and the largest is 
61.2), and we may conclude that the retardation in water loss, for 
this period, is about 49 per cent. It is improbable that there 
occurred any closing of stomata during the period involved in this 
calculation, for the maximum in the evaporating power of the air 
always occurred long before sunset. Indeed, the retardation which 
is manifest before any stomatal closing is to be expected is of a 
considerably higher magnitude than that taken in our calculation, 
so that the average value of this retardation is surely somewhat 
greater than that given here. 
With this approximation of the magnitude of the internal 
retardation of water loss, and with the above observation as to its 
time of occurrence in the day (10:00 to 1:00), we may attempt to 
ascertain what may be its probable cause. 
Preliminary considerations 
(1) INCIPIENT DRYING 
In some recent studies? bearing upon the quantitative relation 
between the intensity of sunlight and transpiration, it has been 
emphasized more than heretofore how important an accelerating 
influence sunshine may exert upon the rate of water loss from 
green plants. This influence of solar intensity upon transpiration 
rate led the writer of the paper last cited to suggest that the internal 
retardation in water loss which occurs prior to the maximum rate of 
oe B. E., (x) Light intensity and transpiration. Bot. Gaz. 52:417- 
438. 19 
———, (2) A radio-atmometer for comparing light intensities. Plant World 4: 
96-99. 1911. 
