Copeland . — The Mechanism of Stomata . 329 
have been at its maximum. As my own experiments will 
show, stomata open in bright light when under water. 
Von Mohl’s assumption, that light influenced the stomata 
by photosynthetically increasing the amount of osmotically 
active material in the guard-cells, has usually been concurred 
in by those who ascribed to them the chief activity in their 
movements. Schellenberg 1 maintains that absence of CO, 2 
suspends the action of light ; but Darwin’s 2 results are the 
direct opposite. 
As to the factors which directly affect stomata, it is 
generally agreed that abundant moisture, in the plant and 
in the air, tends to open the pore. When the plant transpires 
too actively the stomata begin to close, before the wilting is 
discernible (Leitgeb, Darwin). Von Mohl’s statement, that 
if the epidermis of most plants, with the stomata open, be 
mounted in water the stomata promptly close, is not true of 
all with motile stomata (for instance, not of Medeola , nor, 
as von Mohl saw, of many Orchids). When it occurs it is 
because the structure of the epidermal cells lets them take 
up water more rapidly than the guard-cells can, and after 
a time the pore reopens (Schaefer, 1 . c., p. 189), with sections 
thick enough to include some of the mesophyll, so that the 
water does not come into immediate contact with the inner 
wall of the epidermis, this preliminary closing is prevented. 
According to Muller, Kohl 3 , and Darwin, closed stomata are 
opened by a rise in temperature ; Schwendener could not 
substantiate this, and Leitgeb found it true in light but not in 
darkness. 
The influence of light is to open the pore, which tends to 
close in darkness, and the closure at night has sometimes 
been regarded as wellnigh universal. The most generally 
recognized exceptions to this rule are halophytes, hydro- 
1 H. Schellenberg, Beitrrige zur Kenntniss von Ban und Function der Spaltoff- 
nungen. Bot. Zeit., liv (1896), 169-185. 
2 F. Darwin, Observations on Stomata. Proceedings of the Royal Society, Ixiii. 
(1898), 413-417. Philos. Trans., Royal Soc., cxc (1898), 531-621 : pp. 608-610. 
8 F. G. Kohl, Zur Mechanik der Spaltbffnungsbewegungen. Bot. Beibl. z. 
Leopoldina, 1895. 
Z 
