The Interrelations of Stomatal Aperture, Leaf 
Water-content, and Transpiration Rate. 
BY 
R. C. KNIGHT, 
Frtm (he Department of Plant Physiology and Pathology, Imperial College of Science and 
Technology, London. 
With four Figures in the Text. 
R ECENT investigators of transpiration have directed their attention 
* towards the relative importance of the various factors controlling 
the rate of water loss from a plant. It was formerly held that the function 
of the regulation of transpiration was efficiently discharged by the stomata, 
and that, therefore, changes of stomatal aperture and of transpiration 
rate ran closely parallel ; on this assumption Darwin based his horn 
hygroscope and differential temperature methods of estimating stomatal 
aperture ( 7 , 8). 
Lloyd (1908) was the first to call in question the completeness of 
stomatal control of transpiration ( 19 ) ; he even went so far as to conclude, 
from the lack of coincidence of the graphs of transpiration and stomatal 
aperture, that stomatal regulation of transpiration did not occur except when 
the stomata were almost, or completely, closed. 
Darwin in 1916 ( 9 ) concluded that the differences between the graphs 
of transpiration rate and of stomatal aperture were not sufficiently important 
to justify the exclusion of stomata as the factor regulating transpiration. 
Livingston ( 17 ) in 1906 attacked the problem in a different manner. 
He attempted to eliminate the influence of atmospheric conditions by 
expressing his results in terms of ‘ relative transpiration \ He found that 
the transpiration rate rose during the early morning hours, but the rise was 
checked before the evaporating power of the air, as measured by the rate 
of water loss from an atmometer, reached a maximum. In other words, 
some factor other than external atmospheric changes (which regulate, of 
course, the water loss of the atmometer) tends to reduce the rate of 
transpiration after a certain time. 
The existence of this inhibiting factor has been demonstrated in various 
plants by Shreve ( 21 ) and Giddings ( 12 ) in a similar manner, and also by 
[Annals of Botany, Vol. XXXI. No. CXXII. April, 1917.] 
