342 Copeland . — The Mechanism of Stomata. 
Achillea millefolium (Figs. 15, 16, 17). 
Achillea and many other plants have stomata strikingly 
like those of Hellehorus , but different in the one very impor- 
tant particular that the breadth of the lumen of the guard-cell 
does not exceed the depth. In the thickening of the walls 
and in the hinges they agree with Hellehorus , and sometimes 
in a moderately oblique dorsal wall as well. The stomata 
of this pattern are all very small, and, while motile, do not 
open wide, and are therefore not good subjects for experi- 
mental study. As I would construe these stomata, they 
operate in part by a stretching of the dorsal wall, as in 
Amaryllis ; but another factor must be recognized, at the 
ends of the guard-cell. The lumen there is deeper than 
broad, and hence widens with increasing turgescence. On its 
ventral face each guard-cell opposes the other, so that the 
increase in breadth must be by the retreat of the dorsal wall. 
When both ends of the dorsal wall are forced backward all 
of it must go, opening the pore ; but as there is a hinge in the 
outer wall only, the inner alone is free to move bodily back- 
ward, and the resulting movement is exactly as in Hellehorus. 
The responsibility of the union of the ends of the guard-cells 
in this mechanism demands a considerable size there, and the 
result is that the pore is short compared with the length of 
the stoma. 
The study of numerous examples has convinced me that in 
stomata of the Hellehorus type the ends almost always act 
somewhat in this same way, except as the direction of the 
axes makes the movement of the dorsal wall be obliquely 
backward and inward. Thus in the stoma represented by 
Fig. 12, the longest axis being a-a', the greatest pressure 
will be at a right angle to it, and the dorsal wall will be 
forced in the direction of (3. W T hile the operation of the 
Hellehorus stoma would be intelligible without this assistance 
from the ends, it is probably no inconsiderable factor. 
Haberlandt is right in recognizing that the type of Hellehorus 
is a combination of those of Mnium and A maryllis ; it may 
