454 



VENTILATING SYSTEM 



correspond to the open, the lighter lines to the closed condition of the 

 stoma). In Hellcborus, according to Schwendener, "it is characteristic of 

 the closing movement that the width of the front cavity remains constant, 

 at any rate within the ordinary limits of internal tension. The back 

 cavity, on the contrary, becomes much narrower, the contraction which 

 results being roughly equal to twice the diameter of the pore-passage 

 (when open) ; the curved dorsal wall of each guard-cell straightens 

 itself and simultaneously revolves around its outer edge, as a door turns 

 on its hinges. The character of these movements is perhaps most 

 readily appreciated, if one expresses them in terms of the change of 

 shape of the guard-cells. When the stoma is closed, the cross-sectional 



Pig. 172. 



Diagram showing a stoma of Heileborus sp. in the open (heavy lines) and in the closed 

 (faint lines) condition. After Schwendener. (From Sachs' Lectures.) 



outline of the guard-cell cavity is roughly that of a scalene triangle. 

 If the internal tension of the cell increases, this triangular figure 

 becomes larger, and at the same time approaches an isosceles form, 

 with the result that the ventral walls draw further apart and the pore- 

 passage is enlarged." 



. Schwendener states that, in Tradcscantia discolor, contraction of the 

 pore-passage is also entirely brought about by an increase in the 

 curvature of the ventral walls, which in its turn depends upon the fact 

 that each guard-cell diminishes in height as its turgor falls. When 

 the stoma opens very widely, however, the dorsal wall and hence, 

 also, the guard-cell as a whole becomes more strongly curved, The 

 stomata of Tradcscantia discolor, and those of certain Liliaceae and 

 Orchidaceae, are evidently constructed on a plan which is, in a sense, 

 intermediate between the two extreme forms of mechanism that may 

 respectively be termed the Amaryllis type and the Mnium type. The 

 recent investigations of Copeland have brought to light still other 

 types of stomatic mechanism. 



