358 Knight . — ‘ Relative Transpiration ' as a Measure of 
Experiment III. 
A shoot of Eupatorium adenophorum and three paper atmometers of 
different sizes were used. When the rates of water loss had been practically 
constant for two hours, the humidity was raised, and after allowing about 
half an hour for the conditions to come into equilibrium the rates of 
transpiration evaporation were again determined. 
The results are given below : 
Table III. 
Time. 
1.0 p.m, 
1-30 » 
2.0 ,, 
2.30 „ 
3 -° » 
3-30 „ 
5 -° » 
Temp. 
0 C. 
Relative 
Humidity 
per cent. 
21*3 
76 
213 
7 6 -5 
21*4 
76 
21-4 
75*5 
21*4 
75*5 
21*5 
80 
21.5 
80*5 
21-5 
81 
21*5 
81 
Plant 
A bsorption. T ranspiration . 
/ 
Loss from 
Atmometers. 
11 III 
mg. 
mg. 
mg. 
mg. 
mg. 
4ro 
395 
97 
180 
321 
410 
390 
98 
175 
322 
410 
397 
97 
J 75 
333 
420 
4°3 
98 
i 75 
330 
960 
920 
224 
401 
00 
Tt" 
Comparing the 90-minute periods 1.30-3.0 and 3.30-5.0 we get : — 
Period. 
Plant. 
Atmometers. 
/ II III 
(1) 1.30-3.0 
1190 
293 
525 
985 
(2) 3.30-5.° 
„ . Period ( i ) 
920 
224 
401 
748 
Ratio . 
Period ^2) 
1*29 
I ' 3 I 
i* 3 T 
1*32 
As in the experiments with temperature, experiments were carried out 
to determine the behaviour of the stomata under varying conditions of 
humidity. The method used was exactly similar to that described for the 
temperature experiments, and, with a change of relative humidity from 
77 per cent, to 84 per cent., the relative stomatal apertures as calculated 
from porometer readings were : 
Humidity 77 per cent.-— max. aperture 96 
min. 
a 
93 
Humidity 84 per cent. — max. 
>) 
95 
min. 
>> 
93 
It is to be concluded, therefore, that changes of relative humidity 
affect the rates of evaporation and transpiration proportionately. 
From the foregoing experiments it appears that changes in ‘ relative 
transpiration ’ do not necessarily represent changes in the intrinsic 
transpiring power of a plant unless conditions of air movement are main- 
tained constant, owing to the unequal response of plant and atmometer to 
