STRUCTURE OF A TYPICAL STOMA 



447 



durchgang). Both the cavities and the pore-passage are bounded by the 

 ventral wall of the guard-cell. Collectively they constitute the orifice 

 or pore, which is thus of unequal width at different points, often appear- 

 ing roughly hourglass-shaped in vertical section. Tschirch terms the 

 entrance to the front cavity the " eisodial aperture," and the exit from 

 the back cavity the " opisthial " aperture. 



In this widely distributed type of 

 stoma, the dorsal walls of the guard-cells 

 are entirely unthickened and usually 

 bulge into the cavities of the adjoining 

 epidermal (or subsidiary) cells. 



A stomatic apparatus which changes 

 its outline as seen in surface view 

 when the stoma is opening or closing, 

 obviously cannot be immoveably attached 

 to the comparatively rigid outer epidermal 

 wall ; the latter is, in fact, specially thin 

 in the immediate vicinity of the dorsal 

 wall of each guard-cell, along a more or 

 less well-defined strip, which thus acts as 

 a dorsal hinge (the Hautgelenk of Schwen- 

 dener) during the opening and closing 

 of the stoma. In some cases the hinge 

 only occupies an exceedingly narrow strip 

 of the thickened outer wall (Prunus 

 Laurocerasus,Myrtus communis, Allium Cepa, 



Fig. 168 b), while in other instances it comprises a more extensive 

 region of the cell-membrane (Chlorophytum, comosum Fig. 168 a). 

 Among the Cyperaceae the hinges are represented by the entire thin 

 outer walls of the subsidiary cells. In the first-mentioned case the 

 movement of the guard-cells consists in a simple rotation around the 

 hinge ; in the instance last referred to, on the other hand, the 

 expansion and contraction of the orifice involves outward curvature or 

 straightening of the thin strip of membrane. In a number of plants, 

 the epidermal cells, or, at any rate, the subsidiary cells of the stomata, 

 are provided with more or less thickened inner walls. The author has 

 shown that, where this is the case, these walls are likewise furnished 

 with thin strips in the immediate neighbourhood of the guard-cells. 

 The strips in question may be termed inner hinges, in contradistinction 

 from the outer hinges which have been described above. In the leaves 

 of Ghlorophytum comosum, Olivia nobilis, Uropetalum serotinum, etc. 

 the inner hinge takes the shape of a narrow, very well-defined strip. 

 (Fig. 168). In Linum usitatissimum., and among the Bromeliaceae, 



Fig. 167. 



Stoma of Narcissus bifiorus. A. In 



surface view. B. In T.S. V. Front 



cavity. C. Pore-passage. H. Back 

 cavity. 



