452 



VENTILATING SYSTEM 



one direction, but otherwise regular. When the stoma opens or closes, 

 the two longest sides of the hexagon undergo the greatest displacement, 

 always, however, remaining parallel to their original direction, for 

 reasons which have been explained ; as Schwendener remarks, the 

 mechanism is comparable to that of the adjustable slit of a 

 spectroscope. 



In a few grasses (Cynosurvs echinatus, Aira capillata, Brim 

 maxima) the pore-passage remains open, even after the guard-cells 

 have been killed and are therefore non-turgescent. If such stomata 



Fin. 170. 



Diagram of one-half of a Grass-stoma in the open (right) and in the closed (left) con- 

 dition. Surface view. (For explanation see text.) After Schwendener. 



are functional at all, closure must be effected by an increase in the 

 turgor pressure of the subsidiary cells. 



All the forms of stomata that have been discussed so far, agree in 

 the fact that the width of the whole structure increases as the pore 

 opens. The type which still requires to be described, on the other 

 hand, is characterised by the circumstance, that both the total width 

 of the stoma and the shape of the dorsal guard-cell walls remain 

 unaltered, either throughout the opening (or closing) movement or, 

 more frequently, during a considerable part of the movement. The 

 changes in the shape of the guard-cells, which regulate the dimensions 

 of the pore in such cases, must be quite different from any that have 

 been described in connection with other types of stomata. 



The simplest, and at the same time the most extreme, type of this 

 " constant-width " mechanism is exemplified by the stomata on the 

 capsule of Milium cuspidatum (Fig. 171), which have been examined 

 from this point of view by the author himself. The most striking 

 features revealed by a transverse section through such a stoma are, the 



