272 
Notes. 
paradox. In all other cases, the influence of light upon irritable 
cells induces a diminution of turgidity. It is therefore improbable 
that the guard-cells of the stomata should react to light in the 
exceptional manner involved in Sachs’ suggestion. 
Leitgeb’s 1 observations tend to prove that the guard-cells do not 
possess a specific irritability to the action of light. He finds that the 
main determining cause of the opening and closing of the stomata 
is the amount of water in the tissue, and the hygrometric state of 
the surrounding atmosphere. He gives up, in fact, the idea that the 
protoplasm of the guard-cells is irritable at all, and his views have 
since been accepted by Sachs 2 . 
It is not a little remarkable that Leitgeb should have abandoned 
the theory of the irritability of the guard-cells, when some of his own 
observations seem to indicate it so clearly. The reason for this over- 
sight is apparently this, that Leitgeb confined his attention to the 
opening of the stomata, neglecting the closing. 
It is, nevertheless, in the closing of the stomata that evidence in 
support of the irritability is to be sought; for in all cases of move- 
ment known among plants, the movement due to stimulation is 
associated with a diminution of turgidity. 
The particular observations of Leitgeb which seem to indicate an 
irritability of the guard-cells in connexion with the closing of the 
stomata, are those which establish the fact that when a plant, or a 
part of a plant, is brought into a comparatively dry atmosphere, the 
stomata close. It had been previously observed by Amici and von 
Mohl that the stomata of withering leaves are closed, a fact which 
can, apparently, be easily explained on the assumption that, under 
these conditions, the guard-cells, like the other cells of the leaf, 
become flaccid. This explanation is, however, open to the criticism 
that, under these conditions, the epidermal cells, having become 
flaccid, would tend to drag the guard-cells apart, and so to open 
the stomata. Moreover, it is rendered untenable by the fact, 
recorded by Leitgeb, that it suffices to bring a plant from a damp 
to a dry room, to cause closure of the stomata ; in other words, that 
the stomata close when there is no indication of general flaccidity of 
the leaf-tissues. 
1 Leitgeb, Beitrage zur Physiologie der Spaltofifnungsapparate, in Mitth. aus dem 
Bot. Inst. zuGraz, i es Heft, 1886. 
2 von Sachs, Vorlesungen, 2 te And., 1887, p. 231. 
