‘Relative Transpiration’ as a Measure of the Intrinsic 
Transpiring Power of the Plant. 
BY 
R. C. KNIGHT, 
From the Department of Plant Physiology and Pathology , Imperial College of Science and 
Technology , London . 
T HE term ‘ relative transpiration ’ was first employed by Livingston 
in 1906 (8) as the expression for the intrinsic transpiring power 
of a plant. It was defined as the amount of water lost per hour from 
one square cm. of plant surface, divided by the amount lost per hour from 
one square cm. of a water surface under similar atmospheric conditions, and 
this number, of course, represents the area of water surface which would 
evaporate water at the same rate as unit area of the plant. 
Since 1906 the methods of measurement of evaporation and the units 
employed have been modified, but the conception of ‘ relative transpiration ’ 
remains unaltered and has been used by various investigators, e. g. 
Shreve ( 12 ) and Livingston and Hawkins (10). 
The process of transpiration is essentially, of course, the evaporation 
of water from a wet surface, and consequently changes in atmospheric 
conditions, such as temperature, humidity and air movement, which cause 
an increase or decrease in evaporation, also produce similar effects upon 
transpiration. Thus among the factors influencing transpiration an impor- 
tant part is played by atmospheric conditions, and therefore most measure- 
ments of transpiration are markedly affected by these external conditions. 
In an experimental investigation of the effect of any single factor 
on a physiological process such as transpiration it is often desirable that the 
effect of all other factors should remain constant throughout the experiment, 
so that any change of behaviour under experimental conditions may be 
‘ ascribed to its proper source. Thus when experimenting upon the influence 
of, say, light on transpiration, whilst varying the light conditions the 
evaporating power of the air should be maintained constant, otherwise 
changes in the rate of transpiration would not necessarily be the result 
of changes in illumination. 
Under ordinary conditions the evaporating power of the air is constantly 
changing owing to the day and night fluctuations of temperature and 
[Annals of Botany, Vol. XXXI. Nos. CXXIII and CXXIV. July and October, 1917.] 
