134 
R. C. Knight. 
flue for the production of air currents of a constant velocity was 
designed by Professor Blackman and the writer. A description of 
this flue together with some account of its efficiency is appearing 
elsewhere (5). Having thus provided a means for controlling 
air movements in the region of the plant, or other evaporating sur¬ 
faces, it was possible to test the value of the relative transpiration 
method. Experiments were conducted to determine the effect of 
variations of temperature, relative humidity of the air, and speed 
of movement of the air on the rates of evaporation of water from 
plant surfaces and from various atmometers. The results showed 
that relative transpiration is not necessarily a correct expression of 
the intrinsic transpiring power of a plant unless the velocity of the 
movement of the air is maintained constant (see Knight, 15). 
The precautions and conditions which the foregoing work had 
shown to be necessary were adopted in planning and carrying out a 
further series of experiments in which records were obtained of 
temperature, of relative humidity, of rates of water-loss from plants 
and from atmometers of various types, of stomatal aperture and 
also of the water content of the plant under observation, as indi¬ 
cated by the difference between the amount of water absorbed and 
the amount lost by transpiration. 
The experiments were conducted under various atmospheric 
conditions, involving both the normal daily fluctuations operating 
in a greenhouse, and the closely controlled environment which it 
is possible to maintain in a dark room. In all cases the air flue 
mentioned above was employed to ensure constancy of air move¬ 
ment. The writer concluded as the result of this work that both 
the stomata and the water content of the plant were important 
factors in the control of transpiration. It was found that the 
stomata did not, however, close in response to slight reductions 
of the water content of the leaf, but that they were far more sensitive 
to changes of illumination. On the other hand slight deficiencies 
in the water content were effective in reducing the rate of tran¬ 
spiration, and it was found possible to increase the rate of 
transpiration of a plant under controlled conditions by producing 
experimentally an increase in the water content of the cells. This 
work thus gave results which are in full agreement with those 
of Livingston and Brown and of Mrs. Shreve (29). Trelease and 
Livingston (30), using the porometer method, found that the 
stomata continued to open after the transpiring power of the 
plant had reached a maximum for the day, and from this and 
their earlier results they concluded that the reduction of the 
