Copeland . — The Mechanism of Stomata. 36 r 
And to some extent the conditions under which the various 
types are advantageous have already been indicated. The 
stomata of Mniunt , Funaria , Equisetum , the Gymnospermae, 
and the Glumiferae have so limited and well-defined a range 
of occurrence that it may reasonably be supposed that on 
inner grounds which have become very firmly hereditary 
they are not now among the devices possible to most plants. 
The four types of Amaryllis , Medeola, Helleborus, and Achillea 
have so wide and discontinuous a distribution that it is evident 
that in the development of individual orders or genera, or even 
species, there is often a possibility for selection among them. 
When the environment and structure of a plant are such that 
it is desirable that the stoma be very sensitive, then the guard- 
cells must be large and have thin walls, so that they can 
transpire and lose their turgescence before the rest of the 
leaf feels the loss of water. Such stomata are of the Amaryllis 
and Medeola types, especially the latter. Very likely the 
character of the epidermis most frequently determines the 
choice between these two types: if it is deep th t Amaryllis 
type with its deep guard-cells will be natural ; if shallow, 
the shallow Medeola guard-cells. Beyond this, it may 
perhaps be said that the Amaryllis stoma demands large 
neighbouring cells which can readily accommodate the move- 
ment of the guard-cells. And when the epidermis contains 
chlorophyll, as in most of our Ferns, its turgor may vary 
somewhat with that of the guard-cells ; the Medeola structure 
makes the turgor of other cells indifferent mechanically, 
though it may still influence the supply of water to the 
guard-cells. The Medeola stoma has the least area of contact 
with other cells, by which it can receive water, and is therein 
most sensitive. Stomata of th z Flelleborus and Achillea types 
have the free walls of the guard-cells so reduced in area and 
so thickened that transpiration from them is not likely to 
close the pore so long as the mesophyll is well supplied 
with water. They will occur on plants which guard against 
loss of water sufficiently in other ways, so that too great 
sensitiveness of the stomata might result in inadequate 
B b 
