Copeland. — The Mechanism of Stomata. 341 
the dorsal wall is straighter and perpendicular to the surface : 
as the inner wall moves backward and inward the pore of 
course opens. The inner wall of the subsidiary or other 
contiguous cell is usually thin throughout, and attached to 
the inner dorsal angle of the guard-cell, more or less at 
a right angle to the surface. This provides that it shall offer 
no considerable resistance to the movement of the guard-cell ; 
and also, as Schwendener (1889) observes with reference to 
the Gramineae, by giving the deep subsidiary cell a free and 
elastic inner wall, removes its turgescence as an obstacle to 
the movement of the guard-cell (cf. Fig. 55 ). 
It has usually been assumed that a hinge belonged to the 
wall of the subsidiary cell. Haberlandt (1. c., 1886, p. 466 ) 
says that a hinge is often present on the outer wall of the 
guard-cell of Mosses, near the dorsal side ; but the rest of 
the description — walls inequally thickened, thicker ventrally, 
and back walls more or less thin— fits the type of Amaryllis , 
with which a hinge in such a place would be useless. 
Linsbauer (1. c., p. 1000 , Fig. 7 ) figures the guard-cell of 
Lycopodium Phlegmaria with a hinge on the inner wall. In 
stomata of the type of Hellehorus , as the pore opens by an 
inward and downward movement of its sides, the hinge 
representing the axis of a cylinder in whose periphery the 
side of the pore moves, it is evident that the nearer to the 
ridge of entrance the hinge is placed the more the sides of 
the pore will move backward, and the less, inward. Accord- 
ingly the hinge is practically always brought forward on to 
the outer wall of the guard-cell, the dorsal part of which 
remains thin for this reason. This is well illustrated by 
Ipomoea hederacea (Figs. 54? 55) • The same advantage is 
gained if the pore is situated deep down in the rift, as far 
as possible inside the level of the hinge. In Helleboms and 
other stomata of its type the pore is therefore well inside the 
middle of the rift, almost down to the beginning of the inner 
wall. This principle is carried still farther in the stoma of 
Viburnum prunifolium , L. (Fig. 14 ), in which closure is 
effected by the ridge of exit. 
