Recent Work on Transpiration. 137 
and apparently without precautions as to regulation or determina¬ 
tion of the influence of the changing evaporating power of the air. 
By the use of atmometers and the air-flue, and by using Darwin’s 
own method of eliminating the influence of stomatal change by 
blocking the pores with vaseline, and then slitting the leaves to pro. 
vide a passage for the evaporated water to reach the outer air, it 
appears probable that a more exact idea may be obtained of the 
effect of light on the evaporating power of the mesophyll cells. 
Also artificial light can be used and experiments can be carried 
out in a dark room where temperature and humidity are practically 
constant, thus dispensing with the use of atmometers. 
With the same apparatus and a slightly modified method it 
will be possible to investigate the effect of light on various tissues. 
The writer has in view a few plants accessible to him (which have 
been discovered by a lengthy process of trial and error) from the 
leaves of which the epidermis can be easily removed, leaving the 
mesophyll exposed. The effect of light on the rate of evaporation 
from the mesophyll tissue can thus be determined as in Darwin’s 
experiments. Another possibility which may be of use if other 
results warrant its adoption, is the employment of the tissue of 
potatoes, the outer layers of which have developed chlorophyll as 
the result of prolonged exposure to light. 
The same method offers the possibility of investigating the 
peculiar power of some cacti to transpire more by night than 
during the day (cf. p. 130 above). The application of the evapora¬ 
tion method along the lines indicated above should be of value in 
this connection by determining the resistance of the tissue to 
evaporation into the air instead of its power of absorption on 
immersion in water, which was the principle' of Mrs. Shreve’s 
work (28). 
A considerable amount of work has been done on the 
mechanism of the movements of stomata, and a number of 
observations of stomatal behaviour have been recorded which have 
not yet been fully explained. The relation of the water content of 
the leaf to the size of the stomatal aperture has in recent years 
been discussed (see p. 130), and 11 jin offered the outline of an explana¬ 
tion of the manner in which excessive transpiration might affect 
the stomatal aperture, but no evidence has been adduced in sup¬ 
port of his theory that the enyzmes of the cells may be actuated by 
changes in the water content. Lloyd (23) has also dealt with the 
relation of the size of the stomatal aperture to the contents of the 
guard cells. 
