352 Knight. — ‘ Relative Transpiration as a Measure of 
humidity and the more or less irregular changes in the rate of air movement ; 
so that under greenhouse or field conditions it is almost impossible to keep 
the evaporating power of the air constant. To avoid this difficulty 
Livingston made use in his experiments of the conception of £ relative 
transpiration’. His method was to measure, in addition to transpiration, 
the rate of evaporation of water from an atmometer placed near the plants 
under observation, and to consider only changes in the relative rate of these 
two processes as of physiological importance. 
He assumed that by this means the direct effect of atmospheric 
conditions on transpiration could be neglected in the interpretation of 
his results. In other words, any changes in the relative rates of transpira- 
tion and of evaporation, i. e. changes in the 6 relative transpiration ’ of 
a plant, could be attributed to changes of internal factors, that is, to factors 
other than those controlling the evaporating power of the air. 
The assumption underlying this principle is that changes in the 
atmospheric environment affect equally the rate of transpiration of a plant 
and the rate of evaporation from a water surface. This assumption is, how- 
ever, hardly warranted by a consideration of the facts. With reference to 
the standardization of various types of atmometer, Livingston ( 7 , p. 22) has 
pointed out, and Briggs and Shantz ( 3 , p. 636) have emphasized the fact, that 
atmometers of different sizes and shapes are not really comparable under 
changing conditions. For example, a change of environment which doubles 
the rate of evaporation from one atmometer does not necessarily double the 
rate of evaporation from another of different size or shape. In exactly the 
same manner the size and shape of the plant may modify the influence of 
environmental changes on its rate of transpiration. Renner ( 11 ) has 
indicated the part played by the size and shape of leaves in the response of 
transpiration to external conditions. He found that while under the influ- 
ence of a considerable air movement transpiration is proportional to leaf 
area, as the speed of the air diminishes transpiration becomes more and 
more nearly proportional to the diameter of the leaf. Therefore in compara- 
tively still air a small leaf transpires relatively more than a large leaf. This 
is to be attributed to the changes brought about by wind in the nature of 
the diffusion stream extending from the evaporating cells to the drier 
external air. 
It appears, therefore, that it is not possible to compare either one 
atmometer with another or one leaf with another under changing environ- 
mental conditions, so that it seems hardly justifiable to compare an 
atmometer with a plant. 
A further a priori criticism of the • relative transpiration ’ method 
is to be found on consideration of the structure of the leaf. A portion of 
the path along which the diffusion stream passes from the evaporating 
mesophyll cells to the outside air, is situated actually within the leaf, 
