Copeland . — The Mechanism of Stomata. 333 
already familiar — those of Amaryllis , Helleborus and the 
Gramineae 1 > described by Schwendener, Haberlandt’s Mnium 
type, and the peculiar case of Azolla as explained by Schaefer — 
will be taken up in appropriate places in a sequence intended 
to show their relations to one another and to the various 
other forms to be described. 
When we say that the guard-cell becomes more turgescent 
we mean that its volume is forcibly increased. As the cell 
begins to take up water, the pressure against all units of area 
of the wall is the same. Therefore the greater walls sustain 
the greater pressure ; or in other words, the greatest pressure 
is exerted in the direction of the least diameter of the cell. 
To bend a wall requires less pressure than to stretch it, 
because in bending only one of its faces is stretched. If, 
then, the wall of an anisodiametric guard-cell be reasonably 
thin throughout, or in large part, an increase in turgescence 
will be accommodated without any stretching of the wall, by 
an equalization of the diameters, the cell becoming more 
nearly spherical. Stomata meeting these conditions should 
be regarded as structurally simple because the lack of great 
local thickening leaves them in so far like cells of the epider- 
mal tissue. We will consider first stomata of little depth, 
which enlarge, with increasing turgescence, at right angles to 
the surface of the leaf. 
Medeola Virginica, L. 
The stomata of Medeola are irregularly scattered over both 
surfaces of the leaves. The epidermal cells are very large, 
with remarkably sinuate walls. The stomata are likewise 
large, of rather irregular outline, usually not quite as wide 
as long, though occasionally wider, as in Fig. 1. The very 
obtuse angles making the irregularity seen in surface view 
mark either the points of connexion with the stoma of the 
walls between the epidermal cells, or more often peculiar out- 
growths or folds of the wall into the lumen of the epidermal 
1 S. Schwendener, Die Spaltoffnungen der Gramineen und Cyperaceen. Sitzber. 
Berl. Akad. Wiss., 1889, i, 65-77. 
