42 
exceedingly shallow, namely, 3 p deep in a stoma 44 p by 31 p, but it is thick- 
walled. Whatever motility these cells may have is like that possessed by 
Osmunda, 17 the activity being restricted to the tliin-walled ends, which open the 
pore by an increase in depth. Finally, the stoma of Dipteris is almost exactly 
like that of the Coniferw and Allium, with a very oblique dorsal wall and large 
overlying twin subsidiary cells. 
It is obvious in regarding the occurrence of the different mechanical 
types -that those the movement of which involves' a change of outline 
can operate well only where the rest of the epidermis is not too rigid ; 
while those the movement of which involves no change of outline can 
operate in spite of rigid neighboring walls, and indeed are often protected 
by such walls from interference with them by the neighboring tissues. 
These stomata (types of Medeola and Mnium ) regulate the openness' 
of the pores by changes in the depth of the guard cells, and these changes 
are effective because the stomata are broad. The firm structure of epi- 
phytes therefore puts a premium on broad stomata. The ratio of average 
length to average width of the stomata of all San Eamon epiphytes is 
1 to 0.67 ; of terrestrial species, 1 to 0.47. 
Even apparently differentiated subsidiary cells practically never occur 
with the stomata of the Medeola and Mnium types, which are inde- 
pendent of the contiguous cells. However, in stomata the movement of 
which involves any accommodation on the part of the adjacent cells, as 
is the case in stomata of the types of Amaryllis, Dennstcedtia and the 
Coniferce, specialized subsidiary cells are or are not necessary, according 
to the nature of the general epidermal cells. If the latter are very large, 
as in Diplazium, and without too rigid walls, a specialization of subsidiary 
cells seen in surface view or in section is unnecessary and does not 
occur. Subsidiary cells are without one or the other of the two prop- 
erties of the majority of epidermal cells which interfere with the 
movements of the guard cells; that is, either the subsidiary cells contain 
less chlorophyll than the epidermal cells, or they have less rigid walls, 
or both. They contain less chlorophyll in Aspidium angustatum and 
Lindsaya pulchella and none at all in any Antrophyum, in all species of 
which genus other epidermal cells contain a certain quantity. It has 
already been pointed out that wavy, anticlinal walls increase the rigidity 
of the epidermis. An epidermis wholly wavy or wavy next the outer 
wall (as in Antrophyum ) has subsidiary cells with plane walls in Cyclo- 
peltis, Nephrodium syrmaticum, Aspidium angulatum , Arthropteris, 
Pteris ensiformis (not always), Antrophyum latifolium, etc., and Photi- 
nopteris (not always). Humata heterophylla has the anticlinal walls 
which strike the backs of the guard cells very thin, while elsewhere they 
are thick. Mono grama has conspicuously broad, subsidiary cells with 
the narrow stomata. Dipteris has small, angular epidermal cells rich 
1T Copeland, 1. c. 347, figs. 31 to 33. 
