May a8,1917 
Transpiration Rate of Medicago sativa 
287 
with the probable error. In other words, the relative evaporation rate 
through a 24-hour period is practically the same whether the porous 
evaporating surface is in the form of a closed vertical cylinder, a sphere, 
or a fiat horizontal plate. The brown cylinder, owing to its higher 
evaporation rate during the daylight hours, reduces the ordinates of the 
ratio graph; but otherwise this graph is also similar in form to those 
obtained with the white porous surfaces. When the transpiration of the 
plant is referred to the evaporation from a free water surface in a shallow 
blackened tank, a different form of transpiration-evaporation graph is 
obtained, which shows less daily fluctuation than when the transpiration 
is referred to the porous atmometers. 
uu/.y 20 ,/ 9 /e c /c/iy 2 /,/ 9 /e 22,/9/e 
S / 2 M 6/2 6 / 2 A*. 6/2 6 /SAT. 6 /2 
Fig. 3. —Graphs showing the transpiration-evaporation ratio: A, The ratio of the hourly transpiration to 
the hourly mean evaporation from the atmometers. B, Graph A with ordinates plotted logarithmically. 
C, The ratio of the hourly transpiration to the hourly evaporation from the shallow tank, with ordinates 
plotted logarithmically. D, Graphs B (heavy line) and C (light line) superimposed. 
These two types of graphs are compared in figure 3. The first graph 
(A) of the figure represents the ratio of the transpiration rate to the mean 
evaporation rate of the four types of atmometers (16 instruments in all). 
This is the form in which the transpiration-evaporation graph is usually 
presented. It is however preferable, in the opinion of the writers, to 
plot the logarithms of the ratios, as this avoids the distortion of the 
graph, especially when ratios on both sides of unity are being consid¬ 
ered. In the second graph (B) in figure 3, the transpiration-evaporation 
ratio, as given in the first graph, is plotted logarithmically. The third 
graph (C) represents the transpiration-evaporation ratio when the evapo¬ 
ration from the shallow tank is used as a basis of reference, and is also 
plotted logarithmically. At the bottom of figure 3 the last two graphs (B 
