Copeland . — The Mechanism of Stomata . 339 
the presence of this wall is inessential, and spared me the 
obligation to give statistics in this case of Sagittaria by making 
an apparatus representing a stoma of 
Azolla Caroliniana 
on the same mechanical principles, except that it had corners 
which were withdrawn as the sides bulged out. It is obvious 
that only a widening of the middle part of the stoma will 
result in opening the pore. An increase in the width of the 
ends would be disadvantageous, and the thick walls at the 
ends of the guard-cells of Sagittaria , as compared with their 
other walls, may be construed as to prevent this. The stoma 
of Canna has the ridges a little more thickened than those of 
Sagittaria : according to Weiss 1 , its ratio of breadth to length 
is 1 : 3*49. His table contains three others more slender than 
that of Sagittaria , but I cannot speak for their mechanism. 
The ratio of axes of the stoma of Luztda campestris is 1 : 2-84. 
It and that of Lilium superbum represent a transition to the 
type of 
Amaryllis. 
In the stomata hitherto considered some stretching of the 
walls was possible, and in Sagittaria undoubtedly occurred. 
In the type to which we have now come, the stretching of 
the dorsal wall is the most conspicuous feature. I can add 
nothing to Schwendener’s exposition of its mechanism (1. c. 
1881 ). The stomata of this type are broader than that of 
Lilium superbum , deep, usually large, and freely motile. 
Amaryllis formosissima (Schwendeners subject) is stated by 
Weiss to have the ratio of breadth to length 1 : 0*93, it being 
the broadest stoma in his table ; but his material must have 
been unusual in that respect. The thickening of the wall 
of the guard-cell is almost confined to the angles of the 
ventral face, so that with increasing turgescence the dorsal 
wall alone will be stretched and become more convex, draw- 
ing open the pore as in the case of Sagittaria. The stoma 
1 A. Weiss, Untersuchungen iiber die Grossen- und Zahlenverhaltnisse der 
Spaltoffnungen. Jahrb. f. wiss. Bot, iv, 196. 
