Recent Work on Transpiration. 135 
transpiring power was due to the lack of water in the leaf and 
not to any stomatal control. Lloyd (24) found that the stomata 
were often opening when the water content of the plant was 
decreasing. In the experiments mentioned above, the present 
writer also frequently observed the same phenomenon, and inci¬ 
dentally, that the rate of transpiration was decreasing, i.e., the 
reduction of transpiring power was due to the lack of water, which 
was more effective in altering the rate of transpiration than the 
opening of the stomata. At the same time it was observed that 
under suitable conditions it was possible for the transpiring power 
to increase while the stomata were closing, which was the result 
ecorded by Trelease and Livingston. 
The general inference from these results is that when the 
supply of water to the transpiring cells is sufficient to replace that 
lost by transpiration, the movements of the stomata may be of 
primary importance in the regulation of the rate of water-loss, 
as Darwin maintains, but when the rate of transpiration is high 
and the supply of water is insufficient to cope with the demand, 
then lack of water is a more important factor than stomata 
changes. The writer also agreed with Lloyd (24) that the stomata 
do not help to conserve the water of the plant by closure in 
response to slight reductions in the quantity of water in the cells. 
The results outlined above are those which have been pub¬ 
lished up to the present, and it is proposed to give an indication of 
the lines along which the research has since progressed, and also of 
the methods and work which the writer contemplates when circum¬ 
stances permit a return to the laboratory. 
The direct influence of illumination changes on the rate of 
transpiration as indicated by Darwin’s results (10) is one problem 
which has been attacked. Darwin found that when the influence 
of changing stomatal aperture is excluded, the rate of evaporation 
from the mesophyll cells is about 30% greater in daylight than in 
darkness, and that therefore this is a contributing factor to the 
well-known high diurnal transpiration rate of most plants. This 
problem was first attacked by the present writer early in 1914, and 
has also more recently been the subject of experiment. The rates 
of water-loss from slices of plant tissue (potato and turnip) in 
light and darkness were measured by weighing, the evaporating 
power of the air being kept fairly constant by means of drying 
agents and an electric fan. Water surfaces, wet filter-papers and 
films of gelatin were used as controls. The results showed that 
the rate of evaporation in the light was certainly not very con- 
