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The Royal Society, London. 
rise at first, and subsequently sink to zero. The interest of this- 
fact is the demonstration of the interaction between the guard 
cells and the surrounding epidermis. The phenomenon is best 
seen in plants with milky juice, but is not confined to this dass. 
The preliminary opening of the stomata occurs in the early 
morning, but not in the evening—a fact which is of importance 
in relation to the mechanism of the nocturnal closure of the 
stomata. 
A diminution of the stomatal transpiration can also be brought 
about by compressing the stein of the plant in a vice, 
a process which is known to diminish the water supply. 1 ) The 
stomatal closure is here probably an adaptive response to the 
lowering of the water-supply of the leaf, but this is not quite 
certain. 
A series of experiments were made on the comparative effect 
of moist and dry air, from which it is clear that the stomata 
„close“ before any visible signs of flaccidity occur in the leaf. 
When leaves are exposed to air dried by H2SO4, „closure“ is 
preceded by a remarkably prolonged opening of the stomata—a 
phenomenon which requires further investigation. 
Baranetzky 2 ) showed that slight degrees of disturbance affect 
transpiration. The hygroscope gives no evidence of increased 
transpiration when the disturbance is slight. When the plant is 
violently shaken the leaves become flaccid and the stomata „close“, 
and in some cases the closure is preceded by increased tran¬ 
spiration, no doubt due to temporary opening of the stomata, 
induced by the guard cells being released from epidermal 
pressure before they have lost their own turgor. 
N. J. C. Müller 3 ) showed that stomata may be closed by 
electric Stimulation; my experiments show that while a strong 
shock narrows the stomata, a weaker one opens them, no doubt 
owing to the temporary loss of epidermal pressure. 
Some experiments on poisonous gases und vapours were made. 
Chloroform and ether slowly „close“ the stomata, which finally 
reopen in a normal atmosphere. Pure CO 2 also slowly closes the 
stomata. 
The hygroscope is well fitted to demonstrate the fundamental 
facts in relation to light. The fact that the stomata are widely 
open in sunshine is well known; the difference between bright 
and less bright diffused light is not so well known, nor the fact 
that in dark stormy weather the stomata may be nearly closed 
by day, even in summer. The effect of difference of illumination 
is well shown in certain leaves having stomata in both surfaces, 
e. g.. Iris, Narcißsus , and the phyllodes of Acacia cyciopis. In 
these the stomata on the illuminated surfaces are much wider 
open than on the less brightly illuminated sides; and when the 
x ) F. Darwin and R. Phillips, Camb. Phil. Soc. Proc. 1886 . 
2 ) Bot. Zeitung. 1872. 
3 ) Pringsheim’s Jahrbücher. Vol. VIII. 1872. 
