Recent Work on Transpiration. 129 
of the changing evaporating power of the air. “ Relative tran¬ 
spiration is the ratio between the actual transpiration rate and 
the rate of loss of water from a standard evaporating surface 
under the same conditions as the plant. The assumption involved 
in this method is that the rate of the loss of water from the 
evaporating surface (atmometer) is influenced by the changes of 
the external conditions in the same manner as, and proportionately 
to the transpiration rate. It has been pointed out elsewhere that 
this assumption is not necessarily correct under all conditions (15). 
Livingston in 1913 adapted Stahl’s cobalt paper method of 
measuring transpiration rate (20), and by this means controlled 
the external conditions influencing transpiration. This method 
with the addition of more recent improvements has been frequently 
used (1, 2, 3, 22, 30), and appears to be very satisfactory. 
Briggs and Shantz (6), in an extensive research conducted 
under field conditions, have dealt with the external factors affecting 
transpiration. Their method was to observe the plants through 
the normal daily cycle without attempting any control of environ¬ 
ment. Records were kept of all the external factors, and their 
individual influence on the transpiration rate was calculated. The 
results showed that some of these factors were inter related, and 
also that there existed a factor (probably internal) which had not 
been taken into consideration. The authors were of the opinion 
that this factor was either stomatal control or the water supply to 
the leaf. 
The size of the stomatal aperatures has long been recognised 
as a factor concerned in the regulation of the rate of transpiration, 
but the extent to which this regulatory function is shared by the 
stomata with other internal conditions of the plant is a question 
which has been the subject of some controversy. Darwin (7) con¬ 
sidered that the stomata play a predominating part in the control of 
transpiration, whilst Lloyd (23) thought that their importance was 
small. The question has been dealt with more fully elsewhere by 
the present writer (12). 
The water content of transpiring cells is also a factor which 
may influence the rate of transpiration. Livingston and Brown (21) 
concluded that the internal factor, which Livingston (19) 
had previously suspected to be the cause of the reduction of the 
transpiring power of plants in the morning hours, was the lack of 
water in the cells of the leaf. More recently Mrs. Shreve (29) has 
obtained similar results, and entirely confirmed the work of 
