Recent Work on Transpiration. 131 
abnormal state of affairs. Experiments on the rate of transpira¬ 
tion, the rate of absorption, aad the changes of stomatal aperture 
were carried out, and it was found that the stomata were more 
widely open during the night than during the day. The capacity 
of the internal tissue to hold water was determined by placing 
in water, cylinders cut from the fleshy stems and then, by weighing, 
the maximum amount of water which the tissue would contain was 
determined. The results showed that these internal cells were 
capable of holding more water during the day than during the 
night, that is the resistance offered by the plant to the drying 
tendencies of the environment is greater by day than by night, 
presumably owing to the differences of illumination. The author 
suggests (p. 117) that the abnormal changes may be explained by 
the alterations of the water-holding capacity of the internal cells. 
During the day, when the internal tissues have a greater tendency 
to hold water, the outer cells in the epidermal region have a 
correspondingly greater difficulty in obtaining water from within to 
replace that lost by transpiration. The result is a decrease in the 
water content, and therefore of the turgidity, of the epidermal cells 
With the loss of turgidity the stomatal guard-cells collapse, and 
the size of the pore is diminished. Mrs. Shreve thinks that it is 
possible to regard the stomatal changes which were observed, as 
being of sufficient magnitude entirely to account for the unusual 
cycle of the changes of the transpiring power (p. 115). Thus, if 
Mrs. Shreve’s suggestion is correct, the variations of water-holding 
capacity, and therefore of water content, become effective indirectly 
by causing stomatal variations. This is a point of view differing 
slightly from that of Darwin, and also from that of Livingston and 
Brown. Darwin (10) thought that changes of illumination had a 
direct effect on the capacity of the leaf cells to retain water, and 
that this altered the amount of water passing into the atmosphere 
from the plant, independent of any other variations brought about 
by stomatal changes. According to Darwin’s theory the effect of 
light is to increase the rate of transpiration, whilst Mrs. Shreve 
postulates a lower rate of water loss in the light. It must be 
borne in mind, however, that in the latter case the plant con¬ 
cerned was one of abnormal behaviour. Livingston and Brown (21) 
conceived that the change of the evaporating power of the plant 
tissues was also directly affected by variations in the cell water- 
content, again independent of any change of stomatal aperture. 
Mrs. Shreve’s explanation of the phenomena noted in the case 
of Opuntia may suffice for the particular group of plants, but it 
