Copeland — The Mechanism of Stomata. 343 
also contain elements of that of the Gramineae. Only the 
(dynamic) characters of the types of the Gramineae and of 
Amaryllis are present in that of Achillea . In some stomata 
of both of these types, in Aplectrum and Lobelia for instance, 
the inner wall is thickened at least dorsally very nearly to 
the end of the guard-cell. This gives the ends an advantage 
in controlling the middle part of the cell ; and Aplectrum has 
a longer rift in proportion to the length of the stoma than any 
other of its type that I have seen. 
POLYGONATUM BIFLORUM (Figs. 1 8, 19) 
has stomata whose mechanism is such as has just been 
described — probably intermediate between Helleborus and 
Achillea — but which is entirely inverted. The outer wall is 
the more evenly thickened across its surface. The outer 
walls of the adjacent cells are at a right angle to the surface 
of the leaf where they strike the guard-cells ; the dorsal 
walls incline ventrally towards the surface ; and the pore is 
above the middle of the rift. 
The restriction of the active part of the guard-cells to their 
ends, which is partly accomplished in Achillea , is complete in 
the stomata of 
The Gramineae 
and Cyperaceae, which have been thoroughly treated by 
Schwendener ( 1889 ). In median cross-section (Fig. 20) the 
outer and inner walls of the guard-cells are very thick, the 
lumen being reduced to little more than a mere slit. 
The subsidiary cells are much deeper than the guard-cells, 
and have thin outer and inner walls. In longitudinal section 
(Fig. 21) the guard-cells are seen to be enlarged and thin- 
walled at the ends ; cross-sections there (Fig. 22) show the 
depth to be much greater than the width. With increasing 
turgescence the ends of the guard-cells widen, and the retreat- 
ing dorsal wall carries with it the thick, passive middle part 
of the cell, opening the pore. In surface view (Fig. 23), these 
stomata are usually slender and straight-backed. Near each 
end of the guard-cell is a well-marked area, always easily 
