354 Knight. — ‘ Relative Transpiration as a Measure of 
ing power ’ (Bakke, 1 ; Livingston and Hawkins, 10 ) or c transpiration 
coefficient * (Briggs and Shantz, 3 ) of the plant, it indicates that in period 2 
the power of the plant to evaporate water decreased in the ratio 1-97 
to 1*67, in period 6 it increased to 2-03 and decreased again to 1*71 in 
period 8. Taking the means of the respective series, the transpiring power 
in still air is to that in moving air (7 metres per minute) as 2*00 is to 1*71, 
a decrease of 14*5 per cent. There appears to be no reason for this 
T . 
because the maximum variation of in periods 2, 3, 4, 5, 8, 9 and 10, 
when conditions were practically constant, was 1*66-1*75 (less than 6 per 
cent.), and in periods 1, 6 and 7, 1*97-2*03 (3 per cent.). The actual amount 
of transpiration in moving air was about 50 per cent, greater than that 
in still air, and it might be suggested that such a high rate produced 
a deficiency of water within the leaf, resulting in incipient drying of the cells 
(see Livingston and Brown, 9 ) and a consequent reduction of the transpiring 
power of the plant. Consideration of the relation of the quantity of 
water transpired to the volume absorbed in the above experiment, however, 
disposes of this possibility. During periods 2 and 5 the net loss of water to 
the plant (transpiration minus absorption) was 8 and 12 mg. respectively, 
and during periods 3 and 4 the plant gained 8 and 30 mg., i. e. at 2.0 p.m. 
there were 18 mg. more water in the plant than at 12 noon, so that the 
transpiring power should be higher in period 5 than in period i, instead of 
lower. Also a gain to the plant of 30 mg. was accompanied by the 
transpiration of 320 mg. (period 4) and a loss of 8 mg. by the tran- 
spiration of 322 mg. (period 5) and 318 mg. (period 2), so that it appears 
that the incipient drying caused by the small changes of water-con- 
tent indicated above is not sufficient materially to affect the transpiring 
power of the plant. 
There is also the possibility that the movement of the air might have 
caused the stomata to close more completely than they had already done 
in response to darkness, thus producing a corresponding reduction of 
transpiration. No measurements of stomata were made in the experiment 
quoted above, but a series of porometer experiments has been carried out 
both in light and darkness, and it has not been possible to demonstrate any 
significant stomatal change in response to air movements up to a speed of 
20 metres per minute. 
As there is no apparent reason to account for the reduction of the 
transpiring power of the plant, there appears to be no escape from the view 
T 
that the reduction of the rates -= is due to the fact that an increase in the 
h 
speed of air movement accelerates the rate of water loss from an atmometer 
to a greater degree than the rate of transpiration from a plant. Such 
a result is to be expected from the considerations above. 
