Leaf Water-content , and Transpiration Rate. 
237 
12 . 10 2 0 3 0 40 5 0 
NOON P.M. P.M. P.M. p M. P. M 
Fig. 4. Graphs (based on results given in Table IV) showing an increase in the trans- 
piration rate as a result of increasing the water-content of the plant by temporarily decreasing the 
evaporating power of the air. s = stomatal aperture ; E — atmometer loss ; T = transpiration. 
On restarting the fan, transpiration had increased from 400-416 mg. 
per 30 minutes to 465 mg. per 30 minutes. Owing, however, to the 
transpiration rate being again greater than that of absorption, the transpira- 
tion rate showed a decrease in the subsequent periods. 
The stomatal aperture was not quite constant throughout this experi- 
ment, but showed a slight tendency to increase as the experiment proceeded. 
This may have been due to the effect on the stomata of loss of water (see 
p. 234 above). 
Conclusion. 
It appears therefore from the results of these experiments that in plants 
under quite ordinary conditions, changes of stomatal aperture and transpira- 
tion do not necessarily run parallel. Darwin ( 9 ) claims a much closer 
correlation between these two functions than has been obtained in the 
present work, but Darwin made no attempt to control his experimental 
conditions, 1 and most of his transpiration measurements were based upon 
readings of absorption by the plant from a potometer, uncontrolled by 
weighing ; although, as has been frequently shown, absorption is not by any 
1 It should be noted that Darwin’s results were very divergent, as his Tables VII and VIII, 
pp. 433 and 434, show. He compared the increase of transpiration and the increase in stomatal 
aperture in response to light, and this ratio, which in accordance with his theory should be unity, 
varies from 5-5 to less than 0*2. 
