128 K. C. Knight 
the same way as they influence the rate of evaporation, other con¬ 
ditions being equal. 
Renner (26) has carried out a series of researches on the 
influence of various external factors on the rate of transpiration, 
and has emphasised the importance of air movements in connec¬ 
tion with the rate of loss of water from the leaf. He found that a 
wind had a greater effect on the rate of loss from a large leaf than 
on the rate of loss from a small one, and also that in moving air 
transpiration is more nearly proportional to leaf area than is the 
case in still air. He also conceived a “point of saturation” 
existing somewhere within the tissues of the leaf, i.e., presumably 
a point of maximum water content of the air. Renner imagined 
this saturation-point to be sometimes immediately beneath the 
stomatal pore and sometimes deeper in the mesophyll. When the 
point of saturation is immediately beneath the stoma the transpir¬ 
ing system may be considered to consist of the pore of the stoma, 
but when the point of saturation is deeper in the mesophyll, the 
system consists of the pore plus a length of inter-cellular spaces. 
In the latter case a factor independent of the stomata is introduced 
and the part played by the stomata in the control of transpiration 
is proportionately diminished. From his experiments Renner 
concluded that in still air the saturation point is immediately 
below the pore of the stoma, whilst in a wind it is deeper in 
the mesophyll. Therefore in a wind the importance of the stomata 
in the control of transpiration is less than in still air. A con¬ 
sideration of the -path along which the water vapour must pass 
from the transpiring mesophyll cells to the outer air, at once 
shows that the point of maximum water content of the air must 
be at the surface from which the water is evaporating, and that 
the water content must diminish towards the outer air. Renner’s 
general conclusions, however, appear to hold good. 
The same author (27) has carried out some experiments on 
the influence of the external conditions occasioned by the nearness 
of transpiring leaves to each other. He found that the rate of 
evaporation from vertical surfaces was not appreciably affected by 
other evaporating surfaces if they were more than two cms. apart. 
Thus, since external factors which influence the evaporating power 
of the air also influence the rate of transpiration, it is necessary 
when dealing with any of the internal factors affecting transpira¬ 
tion, to control or to eliminate the effect of the external factors. 
Livingston (19) introduced the conception of “ relative tran¬ 
spiration,” the use of which was designed to eliminate the influence 
