344 Copeland. — The Mechanism of Stomata. 
recognized, which represents the thin-walled part of the cell. 
The dorsal and polar walls are usually not far from perpen- 
dicular to the surface, though the dorsal are sometimes 
inclined a little inwardly towards the rift, and the polar in 
the other direction. The inward inclination would give the 
guard-cell an inward as well as a backward movement in 
opening the pore : which will be an advantage when the 
outer wall of the subsidiary cell is considerably thicker than 
the inner, or attached at less of an angle to the surface. But 
the movement of the guard-cells is typically horizontal. 
In the Ericaceae the dorsal and usually the polar walls 
incline strongly ventrally inward ; and in mature stomata, 
when the thickening is such that the median part must be 
quite rigid, they retain a slight motility, which is then 
mechanically of the type of the Gramineae. But, as will be 
shown later, they are. of the type of Achillea when more 
active. Euphorbia dentata , Mx., has stomata on the back of 
the leaf very suggestive of those of the Gramineae, except in 
not being deep at the ends. The deep, thin-walled epidermis 
makes subsidiary cells unnecessary. The stomata of the 
stem are as near Amaryllis as to Achillea , while those of 
the upper surface are intermediate in structure between those 
of the stem and those on the back of the leaf, making a fair 
series of connecting forms on the one plant. 
Among the Gramineae themselves it is common for the 
outer wall of the subsidiary cell to be thicker than the inner, 
as in Fig. 30, and it is always attached at an angle less 
favourable to free movement. The result must be that the 
inner part of the guard-cell is forced backward more easily 
than the outer. If this difference in the ease of movement 
becomes very considerable, the result will be a stoma standing 
in the same relation to the grass type that Mnium does to 
Medeola. The Achillea type will be midway between this 
stoma and that oi Amaryllis, as the Hellebortis is a combination 
of those of Mnium and Amaryllis. 
