362 Copeland. — The Mechanism of Stomata. 
transpiration, as well as interfere with photosynthesis. Of 
the two, the Achillea type, being narrower, has less outer and 
inner wall, and a narrower strip on the ventral wall is likely 
to remain thin. So long as other factors, ecological but not 
mechanical, do not interfere, it is therefore the less sensitive. 
In conclusion, it may prove a convenient summary of part 
of the work in this paper if the mechanical types of stomata 
are concisely tabulated. Of the numerous combinations and 
intermediate forms, there are included here only the two 
which it is convenient to regard as types — Helleborus and 
Achillea. 
The pore opens by — 
1. A change in shape, rather than by stretching the walls, 
in which the change is 
(a) An increase in the depth of the guard-cell, in which 
there is chiefly concerned— 
The entire wall (except the dorsal) . 
Medeola 
( Equisetum ). 
Mnium. 
Funaria. 
Lycopodium. 
Osmunda. 
The inner half . . 
The outer half .... 
The dorsal half .... 
The ends ..... 
(b) An increase in the width of the slender 
stoma 
2. Stretching the thin dorsal wall 
3. A change in shape, with or without much stretching, at 
the ends of the guard-cells which forces the dorsal wall, with 
the passive middle part of the cell — 
Directly backward 
Upward and backward . 
4. Combinations of the types of 
Amaryllis and Mnium 
Amaryllis and the Gramineae 
Sagittaria. 
A maryllis. 
The Gramineae.* 
The Coniferae. 
Helleborus. 
Achillea. 
