340 Copeland . — The Mechanism of Stomata. 
of Smilax glauca , Walt. (Figs. 7, 8), represents this type very 
well. As is true of Amaryllis , the dorsal wall is so thin that 
it readily collapses in glycerine 1 . The stoma of the stem 
of Polygonum sagittatum , L. (Fig. 15), also belongs here; 
the presence of a thin outer wall on the adjoining cell, instead 
of a thick wall with a hinge, makes the stoma more freely 
motile. The stomata of Dracaena sp. and of Euonymus 
atropurpureus , Jacq. (Fig. 10) are also of this type, in that 
the thickening is largely confined to the ventral half of the 
guard-cells, but the thickening is so excessive that motility 
is comparatively limited. 
A great number of stomata combine the features of the 
types of Medeola and Amaryllis , so that in opening the pore 
the depth increases and at least a part of the dorsal wall 
becomes more convex. Most commonly the inner part of the 
stoma executes the movement, the outer walls moving little 
if at all. These are features of the type of 
Helleborus, 
whose mechanism has also been fully explained by Schwen- 
dener ( 1 . c. 1881 , pp. 856-7). The stoma of Aplectrum 
hyemale , Nutt. (Fig. 11), represents this type, which is most 
familiar because, though one of the most complicated, it is 
used in various general texts as a plan from which to explain 
the mechanism of stomata in general. It differs most con- 
spicuously from Amaryllis in that the entire inner wall, as 
seen in median cross-section, is strongly thickened, so that 
it will neither stretch nor bend. The dorsal is thin, somewhat 
convex, and oblique, slanting in and toward the pore. The 
greater diameter of the lumen — not necessarily of the entire 
cell — as seen in cross-section is parallel to the surface : 
accordingly with increasing turgescence the greatest pressure 
is exerted to deepen the cell. This can be done at first 
without stretching any wall, but merely by bending them 
where they are thin, if the inner wall moves backward until 
1 Not however, in my experience, until it has ceased to be stretched ; cf. 
Leitgeb, p. 152. 
