338 Copela?id . — The Mechanism of Stomata . 
an increase in turgescence ; and so long as the total width 
of the stoma does not decrease, the increase in depth con- 
tributes to the opening of the pore. But there are com- 
paratively few whose pore is opened by this factor alone. 
In many stomata of the types next to be considered there 
is some increase in depth, but it is not considerable and may 
be neglected in describing them. 
Sagittaria variabilis, Engelm (Figs. 5, 6). 
The stoma of Sagittaria is conspicuously narrow. The 
one drawn is a fair specimen ; the ratio of width to length 
is i : 2-6. Looking at the stoma as a whole, which I have 
sought to justify by the discussion in connexion with Funaria^ 
it is evident that with increasing turgescence this stoma would 
widen, and if possible shorten. In cross-section it is seen 
that closure is effected by the middle of the ventral wall and 
by the ridge of entrance. If the guard-cell were to be 
regarded as composed of two halves, one of them having 
thick, the other thin walls, the halves would be an outer and 
an inner, rather than a ventral and a dorsal. If now the 
entire stoma widens as its turgescence increases, the outer 
wall, instead of stretching, will draw back the sides of the 
pore, opening it. While the wall between the guard-cells 
does not prevent them from acting as if it were not present 
when the action depends on excess of length over breadth, 
it does, by refusing to stretch so long as an increasing volume 
can be accommodated more easily by a change in shape of 
the stoma, make it necessary to consider the guard-cells 
individually with respect to depth : they are deeper than 
wide. A limited shortening of the stoma is possible without 
compressing or distorting this wall, from the shortening of 
the rift as it widens. Schwendener (1. c. p. 845) says the 
outer vestibule of the stoma of Amaryllis shortens in widen- 
ing; if this does not shorten the entire apparatus, the wall 
between the guard-cells must be singularly elastic. Leitgeb 
reports a considerable shortening of the stoma as it widens. 
Schaeffer (1. c. pp. 199-204) has justified the assumption that 
