EXTERNAL COMMUNICATIONS OF THE AERIFEROUS SYSTEM 193 



FIG. 20. Stoma of Helleborus spec, in the open and 

 closed condition. (After Schwendener.) 



shown how the sectional outline alters as the stoma opens, and how the inner 

 part of the stomatal aperture widens, but not the outer. 



In Graminae and Cyperaceae the apposed ends of the guard-cells are thin- 

 walled (a, Fig. 21), while the edges in contact are thick and rigid (b, Fig. 21). An 

 increase of turgidity causes the end portions to expand and push the rigid lips of 

 the stoma apart from one another. Even the stomata of different moss capsules 

 do not all act in precisely the same 

 manner. Thus, according to Haber- 

 landt, in Mnium cuspidatitm the guard- 

 cells are stretched and elliptical in the 

 direction e-d, as in Fig. 20, but be- 

 come rounded as the turgidity increases, 

 and thus cause the stoma to open. 

 Further peculiarities are to be found 

 in the literature quoted, especially that 

 by Haberlandt and Schafer. 



Even stomata with essentially similar opening mechanisms show various differ- 

 ences as regards the changes of form and position produced by varying turgidity, but 

 these are for the most part of merely accessory importance. Thus in many stomata 

 which open by the method first given, no widening of the internal aperture of the 

 stoma occurs, as it does in Helleborus. Schwendener gives various examples of 

 the antagonistic action of the epidermal cells, but in other cases the guard-cells are 

 loosely attached to the neigh- 

 bouring cells, so that full freedom 

 of movement is assured. 



When those parts of concave 

 walls which touch one another 

 are less thickened, this appears 

 to be of advantage in securing 

 a more complete closure ; at 

 the same time the neighbouring 

 parts of the wall are highly cuti- 

 cularized and strongly thickened, 

 to ensure an adequate protection 

 against transpiration. If the con- 

 cave edge were too thin and 



extensible it would bulge out as the turgidity of the guard-cells increased, 

 and thus partly nullify their action in opening the stoma. The thinness of 

 a portion of the convex outer surface is apparently in order to favour diosmotic 

 exchange with neighbouring cells. 



External agencies which influence turgor will alter the width of the 

 stomatal openings to a greater or less extent, though since different plants 

 and different cells have different powers of reaction, it is not to be expected 

 that the same result should in every case be produced by the same stimulus. 

 The effects produced by changes of illumination indicate this. Thus, 



FIG. 21. Stoma of Avena saliva. A, in face view ; B, a trans- 

 verse section , lirou^jli b ( 



PFEFFER 



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