352 Copeland . — The Mechanism of Stomata. 
S. rupestris , Spring, is much deeper, and with the ridge 
of exit well developed, being in both respects like the 
Amaryllis type. 
The remarkable superficial aspect of the stomata of Eqai- 
setum has made them a frequent subject of note (literature in 
de Bary, 1 . c.), and some unsound speculation. In two 
modern texts it is stated they consist of two pairs of guard- 
cells, one over the other, a notion disproved by a study 
of their development by Strasburger thirty-six years ago 
( 1 . c.j 1866 , pp. 318-22). In cross-section (Fig. 46) it is at 
once evident that the lumen of the guard-cell has two very 
unequal diameters, and will therefore, like that of Medeola , 
tend to become round with -increasing turgescence. The 
ventral and inner walls are practically all one, with a ridge 
of exit too little thickened to affect the mechanism. The 
outer wall is restricted to the ridge of entrance, which con- 
stitutes the pore, and is placed where it looks like a continua- 
tion of the wall between the guard-cell and the overlying 
subsidiary cell. The extensive convex dorsal wall is thin 
except for the thickened strips running across it from the 
pore, projecting into the lumen of the subsidiary cell. These 
strips do not stiffen the wall lengthwise of the guard-cell, 
and will themselves be bent by less pressure than would 
stretch them. Now if the pore be closed — by which I mean 
of course as nearly so as it becomes — and the turgor of the 
guard-cell increases, water will enter it from the subsidiary 
cell, as indicated by the arrow. To compensate for this 
movement of water, the wall must move in the opposite 
direction. This must bend the wall, but need not stretch it. 
Obviously the thickened strips, attached to more than one 
wall and presumably fixed fast at the back side, will exert 
a direct pull on the ridge of entrance and widen the pore. 
There is a conspicuous hinge in the wall above the ridge. 
While the guard-cells are often in contact with no other cells 
than their subsidiary cells, and so must be supplied with 
water through them, this is of course not essential to the 
operation of the stoma. 
