134 
JOHN GRAY AND GEORGE J. PEIRCE 
to show that cuticular transpiration may take place in addition to that 
through the stomata. Thus on applying a film of collodion to the epidermis 
of a leaf, through which the details of the leaf could be seen, it was found that 
clouding took place between the stomata as well as over them, though of 
course not necessarily at the same rate. Applying this fact to the plants 
with which Darwin experimented, it must be seen that a part of Darwin's 
error in recording transpiration was due to the cuticular evaporation of 
which the plants of his region are certainly capable. The cuticular evapora- 
tion of Lloyd's plants, on the other hand, would be very slight, while the 
Gramineae studied at Stanford University, with their moderately cutinized 
epidermis, exhibit a certain amount of cuticular transpiration. The regu- 
lation of transpiration seen to be effected by the stomata would lead one to 
conclude that they directly and mainly control the exchange of gases 
and the outgo of water. Only under exceptionally dry conditions would 
evaporation of water and the transfusion of gases through the outer walls 
of the epidermal cells be of any considerable importance. This we shall 
show shortly. 
Furthermore, the density and the humidity of the air determine the 
intensity and the composition of the light reaching the earth's surface in 
different regions. Unfortunately it is still impossible to express these dif- 
ferences in definite terms, there being no single instrument or set of instru- 
ments which will give us all the data involved; but we know that on a hazy 
or cloudy day there may still be enough light to affect plants noticeably, 
though the sun's rays do not reach them directly. We are forced to con- 
clude that the quantity of light reaching the earth's surface is less in Cam- 
bridge, England, because of the higher humidity of the air, than at Stanford 
University, and still less than in Tucson, Arizona. Some of the bearings 
of this inequality of light distribution have been already discussed by one 
of us elsewhere (6) ; it must be distinctly borne in mind in connection with 
the study of stomatal behavior. For example, the occurrence of high fogs 
at night and in the early forenoon in the San Francisco Bay region marks 
an important difference in the illumination as compared with that where 
Lloyd worked. 
Darwin measured the amount of transpiration and the corresponding 
changes in the stomata by means of a ''poremeter" fitted directly upon 
the surface of the leaf. The variations were estimated from the rate of 
flow of a current of air drawn through the stomata of an uninjured leaf 
under a given pressure. The leaf was supported upon a glass plate and the 
poremeter placed on a washer or perforated disc fastened to the leaf by 
means of gelatine. Others used paraffine, and also illuminated the leaf 
from below. Obviously, stomata enclosed within a poremeter are shut off 
from both light and air; CO2 necessary for food manufacture is shut off 
during the progress of the experiment, and the leaf undergoes a sweating 
process in which water is given off but nothing is taken in, which would 
very rarely, if ever, take place in nature. 
