Copeland . — The Mechanism of Stomata. 349 
The restriction of the active part of the guard-cells to the 
ends in Ophioglossum pendulum is illustrated by Figs. 36 and 
37, made from a stoma which had probably become rigid 
with age. Campbell 1 says of this species, ‘ The upper walls 
of the guard-cells are thickened irregularly ’ : as his figure 
shows, this irregularity consists in leaving the thin areas at 
the ends, familiar in the Gramineae. The same restriction of 
motility to the ends, with a most excessive thickening of the 
middle, characterizes the stoma of Angiopteris (Figs. 38, 39). 
The deep and narrow ends would permit such a movement as 
in the Gramineae, insured more perfectly by the thick lines 
running to the ends : but the insertion of the outer and inner 
walls of the subsidiary cells is such that the outer part of the 
guard -cell, where the ridge of entrance constitutes the pore, 
will execute most of the movement. 
In the Polypodiaceae I have examined there is in all an 
approach to what Haberlandt 2 calls the type of swimming 
plants, in that the ridge of entrance is well developed, while 
the ridge of exit is inconspicuous or not present. In Denti- 
st aedtia punctilobula , Bernh. (Figs. 40, 41), this thickening of 
the ridge of entrance has gone far enough to give the stoma 
a rigid appearance, but it is really motile. Opening seems to 
be effected by a movement of the ridge of entrance outward 
as well as backward, such as must occur in lesser degree in 
the case of Angiopteris. The guard-cells of Dennstaedtia are 
thin-walled and shallow at the ends. In the following table 
the depth is measured from the ridge of entrance down to 
the deepest dorsal focus. The stoma was closed by displacing 
water with alcohol. 
Open. 
Closed. 
Width of stoma 
3 ° 
38-5 
Width of guard-cells . 
13-5 and 14 
14 and 14-5 
Width of pore . 
^•5 
O 
Length of stoma 
44 
45 
Depth 
17 
14 
1 D. H. Campbell, Mosses and Ferns, 1895, p. 
233- 
2 L. c. 1887, Physiol. Pflanzenanatomie, p. 401. 
