Copeland ’ — -The Mechanism of Stomata. 359 
necessary changes in total curvature may not involve an 
impracticable bending of any unit of wall ; and the stomata 
which are surrounded by subsidiary cells formed regularly 
from the same initial cell are from their origin likely to be 
small. The stomata of the Amaryllis type must also be large 
to permit the needed curvature of the entire guard-cell, as is 
of course equally true of Sagittaria. A stoma of the Conifer 
type must be below the general level of the epidermis. Out- 
side the very specialized type of the Gramineae there remain 
only the Helleborus and Achillea types, which on mechanical 
grounds could be expected to occur among the very numerous 
Dicotyledones with small stomata. 
The size of the stoma usually corresponds somewhat to that 
of the epidermal cells (Salvinia is a conspicuous exception). 
And the mechanism of the stoma must be correlated with the 
size and the depth and the thickness of the walls of the 
neighbouring cells. When the subsidiary cells are small, so 
that a small change in volume might demand a difficult 
change in their form, or when anticlinal walls oppose the 
movement of the guard-cells, it is some advantage of the 
Helleborus and Achillea stomata that only the inner half must 
overcome this resistance; and that this inner half has a firm 
wall which can be forced against any single point of resistance 
with the whole force of the turgescence of the cell. 
To some extent more direct ecological adaptations of the 
stomata can be seen; If Dicotyledones are examined in late 
summer, in the height of the season of the large gamopetalous 
orders, as to their stomatal mechanism, the Helleborus or 
Achillea type will be found with scarcely an exception. This 
is true of trees at all times. But if the season of study is 
early spring, when Clay to ni a, Hepatica, Anemonella, Saxifraga 
Virginica , &c. are collected^ the Amaryllis and Medeola 
types will be much in evidence. The Monocotyledones with 
similar stomata are characteristic spring plants. Delphinium 
and Aconitum show a difference that I cannot construe other- 
wise than as an adaptation to the season. The frequent 
difference between the stomata of spring and summer plants 
