Stomata , Leaf Water-content , and Wilting of Plants. 379 
day amounted In one case to more than 20 per cent. (76-6 per cent, at 
10 a.m., 54*7 per cent, at 3 p.m.). Assuming that the dry matter of the 
leaf remained approximately the same, this means that of the original water 
in the leaf about 65 per cent, has been lost. This excessive loss of water is 
very surprising in view of the experiments previously recorded in the present 
paper, especially as Clark makes no mention of wilting. It is possible, 
however, that the leaves used in Clark’s experiments for water-content 
determinations at different times of the day were never really comparable 
at all, but that their respective water-contents differed very considerably at 
the outset. The present paper has shown that considerable differences 
may exist even between apparently similar leaves, and Clark has recorded 
no attempt to determine whether such initial differences existed in the 
plants which she used. The above interpretation of the enormous differences 
found by Clark receives additional support from the fact that she found it 
impossible to associate these variations in water-content with changes of 
temperature or humidity, or with the changes of evaporating power of the 
air which occur at different times of the day. 
Clark also states as a general conclusion that water-content is inde- 
pendent of transpiration, temperature, and humidity. A study of Clark’s 
results shows that this statement means that water-content varies indepen- 
dently of transpiration, temperature, and humidity. The experimental 
evidence offered in previous sections of the present paper indicates that, on 
the contrary, the leaf water-content must remain constant within a very 
narrow range throughout the day. The flow of water into the leaf appears 
to be nicely adjusted to counterbalance the loss by transpiration, and the 
total amount of water in the leaf cells is almost unchanged. Extreme 
conditions, however, whether caused by excessive transpiration or by lack 
of soil moisture, might be expected to disturb this delicate equilibrium, at 
the same time producing flaccidity. In the sense, then, that water-content 
is almost constant and unaffected by variations of transpiration and external 
conditions, it may be said to be independent of these changes, but this 
interpretation of Clark’s statements is quite at variance with her results, 
and is not the one which she accepts. 
The work of Livingston and Brown (17) and of Shreve (19) is relevant 
to the present work. Investigations carried out by these workers showed 
that under desert conditions there was a definite diurnal cycle of changes 
in the water-content of the leaf. A minimum quantity of water was found 
to be present during the period of high evaporation rate, whilst the maximum 
water-content was reached at some time during the night when transpiration 
was low. This would appear to be the natural result of a limited water- 
supply, in consequence of which the passage of water into the leaves is not 
sufficiently rapid during the daytime to replace the water lost by transpira- 
tion. The influence of these conditions upon transpiration rate has been 
