Notes. 
273 
It appears, further, from Leitgeb’s observations, that the dryness 
of the atmosphere is not, in itself, sufficient to cause closure of the 
stomata, but that the amount of water present in, or supplied to, the 
leaves, is an important factor. The closing of the stomata seems to de- 
pend upon a certain relation between the dryness of the air, on the one 
hand, and the amount of water in the transpiring tissue, on the other. 
The problem is one which demands further investigation for its 
complete solution, and I hope to be able to carry out some experi- 
ments in this direction. But I may venture even now to offer some 
suggestions in explanation of the mechanism of opening and closing. 
In the face of Leitgeb’s observations, there can be no doubt that the 
opening of the stomata is not due to stimulation, by light or any other 
agent, of the irritable protoplasm of the guard-cells. The process is 
a purely passive one, depending upon the formation of osmotically 
active organic substances in the chlorophyll-containing guard-cells 
when exposed to light. This view is supported by Leitgeb’s account 
of the opening of the stomata, in which he describes the gradual 
disappearance of the granules in the protoplasm, and of the starch- 
granules in the chlorophyll-corpuscles, in the turgescent guard-cells of 
the opening stoma ; and, further, by N. J. C. Muller’s 1 observation that 
a rise of temperature promotes the opening of the stomata. 
It may be objected that this theory does not account for the opening 
of the stomata of etiolated plants, or for that of normal plants in dark- 
ness. In reply to this objection it may be pointed out that, as Sachs 2 
first demonstrated, the guard-cells of even etiolated plants always con- 
tain starch-granules, and that therefore they have the means of ensuring 
endosmosis and consequent turgidity. 
The closing of the stoma is, on the contrary, an active process, de- 
termined by the stimulating influence of a certain relation between loss 
and supply of water, on the irritable protoplasm of the guard-cells. 
I cannot more clearly state this view than by the following quotation 
from N. J. C. Muller : c Die Oeffnungsstellung ist die normale Stellung 
des Apparates ; die Schliessungsstellung ist die Reizstellung ; ’ though 
the stimuli referred to by Muller are other than the one which I 
suggest. The theory of the function of the stomata which logically 
follows from this view is essentially the same as that originally pro- 
1 N. J. C. Muller, Pringsheim’s Jahrb. f. wiss. Bot., VIII, 1872. 
2 von Sachs, Botanische Zeitung, 1862, and 1863, Beilage. 
