Leaf Water -content, and Transpiration Rate. 
235 
Fig. 3. Graphs (based on the results given in Table III) showing the continued opening of the 
stomata in spite of the increasing water deficiency. The water deficit numbers are plotted below the 
zero line so that a downward trend in the graph represents a decreasing water-content, and vice 
versa. The deficit is assumed to be zero at the beginning of the experiment, s = stomatal aperture ; 
d = water deficit. 
Experiments in Controlled Environment. 
It appeared likely from the results obtained that, if stomatal changes 
and the evaporating power of the air were controlled, the rate of actual 
transpiration might be shown to be dependent upon the water-content of the 
plant, and an attempt was made to do this. 
The experiments were carried out in a dark room where the tempera- 
ture could be maintained constant to within 0-5° C. and the relative humidity 
to within 1 per cent. Trials were made with artificial light in order to 
control the stomata, but it was found that even in the dark the stomata of 
the plant used — E upatorium adenophorum — were fairly widely open and 
that the transpiration rate was still fairly high. Accordingly experiments 
were carried out in the dark, except when weighings were being made, 'when 
an electric lamp some distance from the plants was switched on. It was 
found to have no effect on the stomata. The plants were usually removed 
to the dark room on the day before the experiment was to be performed in 
order to ensure that they might reach equilibrium in the changed conditions. 
