the Intrinsic Transpiring Power of the Plant . 357 
water loss from the atmometers is proportional to the change of transpira- 
tion rate: — 
Period . 
Plant. 
Atmometers. 
(1) 11.30 a.m.-i.o p.m. 
1233 
I 
533 
II 
460 
(2) 1.0 p.m.-2.30 p.m. 
1402 
604 
525 
Period (2) 
Ratio T3 • 1 / \ 
Period (1) 
i*i 4 
i-i 3 
1-14 
( 3 ) 2 - 3 ° p.m.-4.o p.m. 
1536 
671 
578 
Period (3) 
Ratio p . ■ 
Period (1) 
1.25 
1*26 
1*26 
The possibility of change of stomatal aperture was considered, and 
porometer experiments have been carried out to ascertain the behaviour of 
stomata when subjected to changes of temperature of the same order as 
those in Exp. II. A porometer leaf-chamber was attached to a leaf of 
a potted plant in the greenhouse, and stomatal changes were recorded by 
means of the automatic recorder described elsewhere ( 5 ). 1 After a time 
the plant was removed to the dark room, when the stomata closed rapidly. 
When the aperture had become constant, the temperature of the dark room 
was raised by means of the radiator used in Exp. II, and the record of 
stomatal aperture was continued for several hours — in one case eighteen. 
The results showed no significant change of stomatal aperture on raising 
the temperature. The relative size of the apertures, assuming them to be 
proportional to the square root of the reciprocals of the porometer readings 
(see Darwin 4 , p. 423), were as follows in one typical experiment : 
Before raising temperature — max. aperture 92 
min. „ 89 
After raising temperature — max. „ 94 
min. „ 89 
To test the influence of humidity changes upon transpiration and 
evaporation rates, experiments have been carried out on lines similar to 
those of Exp. II above. The plant and various atmometers were exposed 
in the air-flue in the dark room to a stream of air moving at a speed of 
7 metres per minute, and determinations of water loss were made at 
intervals. When the rate of loss had been shown to be constant, the 
relative humidity of the atmosphere was increased by spraying the walls 
and floor of the room with a syringe, the determinations of water loss from 
plant and atmometers being continued. With the rise of humidity the 
temperature tended to decrease and this was checked by the use of the 
radiator. 
1 Since the publication of the paper referred to, the apparatus has been modified so as to 
dispense with the constant temperature bath ; the modified recorder is easily portable. 
