130 R . C. Knight. 
Livingston and Brown. This is also in complete accord with 
the suggestion offered by Briggs and Shantz (see above). 
Some workers have investigated the inter relations of the 
water content of the leaf and the size of the stomatal apertures. 
Darwin (7, 8) and also Darwin and Pertz (9) found that whilst the 
wilting of a leaf finally caused stomatal closure, the first effect of 
loss of water was to produce an opening movement. Lloyd (23) 
failed to find this preliminary opening while Laidlaw and Knight 
(16) obtained results similar to those of Darwin and Pertz. Iljin (11) 
thought that excessive transpiration produced stomatal closure by 
reducing the osmotic pressure of the guard-cells, changes of water 
content serving to stimulate the action of enzymes which brought 
about an alteration in the osmotic substances of the cell. Iljin, 
however, also states that he found open stomata frequently in 
wilted plants, and that the rate of loss of water has no effect on 
the rate at which the stomata close. The first of these statements 
seems to confirm the idea that the stomata may open as the first 
consequence of the loss of water. 
The direct effect of light on the rate of evaporation from leaf 
tissues, as distinct from the indirect effect of light on transpiration 
by the production of stomatal changes, has also been studied. 
Darwin using a new method (10), found that the mesophyll of a 
leaf transpired more rapidly in the light than in the dark. His 
method was to block the stomatal apertures by means of vaseline, 
and then to put the mesophyll in communication with the outer air 
by slitting the leaf in several places. Thus any effect of light 
upon the stomata is prevented from influencing the transpiration 
rate, since the loss of water takes place through the slits and not 
through the stomata. The treatment is rather a drastic one but it 
appears to give consistent results, and Darwin found that the rate 
of evaporation from leaves treated in the manner described was 
more than 30% greater in daylight than in darkness. The experi¬ 
ments, however, were conducted under conditions which were not 
very closely controlled, and it seems desirable to repeat them 
either in a more constant environment or taking into account the 
changes in the external conditions. 
The question of the effect of light on the transpiring pro¬ 
perties of the internal tissues of a plant has more recently been 
investigated by Mrs. Shreve (28) working with Opuntia versicolor 
Succulents of this type are found to have a higher transpiring 
power at night than during the day (Livingston, 17), and Mrs. 
Shreve’s work was directed to discover the reason for this 
