MECHANISM OF A TYPICAL STOMA 440 



of the guard-cells. Schwendener made a number of careful measure- 

 ments, with a micrometer, of the dimensions of the different portions 

 of the stomatic apparatus of Amaryllis formosissima and other plants, in 

 the open and in the closed condition ; in this way he was enabled to 

 gain a complete insight into the changes of shape which the guard- 

 cells undergo, and thus finally to explain the mechanism of the stoma. 

 Closure of the stomata was brought about, in Schwendener's experi- 

 ments, by piasmolysing the guard-cells with glycerine. The following 

 table contains the measurements obtained in the case of Amaryllis 

 formosissima : 



Stoma open. Stoma closed, 



/x \J. 



AVidth of single guard-cell, 37 33 



front cavity, 16 7 



pore-passage, ------ 8 



Distance between the lines of attachment 220 " of the 



two adjoining epidermal cells, 40 34 

 Distance between each line of attachment and the 



dorsal wall of the corresponding guard-cell,- - 19 16 



Consideration of these figures at once shows, that the whole stoma 

 is much wider in the open condition than when it is closed, whereas 

 the length of the apparatus undergoes no appreciable change in the 

 process of opening or closing. It follows that the volume of each 

 guard-cell is greatest in the open condition of the stoma. The process 

 of opening, therefore, depends upon endosmosis of water and 

 consequent increase of the hydrostatic pressure in the guard-cells ; 

 this rise of pressure in turn causes a distension of the dorsal and 

 ventral walls. The thin dorsal wall naturally stretches more readily 

 than the partially thickened ventral wall ; consequently the curvature 

 of the guard-cells is increased and the orifice becomes wider. When 

 turgor falls, on the other hand, the more active contraction of the 

 dorsal wall diminishes the curvature of the guard-cells, and the 

 size of the orifice is reduced. A guard-cell, constructed on the 

 aforesaid lines, will, therefore, behave very much like a short length 

 of rubber-tubing, the wall of which is considerably thickened along one 

 side. Such a tube will curve when water or air is forced into it 

 under pressure, the thick strip forming the concave side. The 

 presence of two specially thickened ridges on the ventral wall of each 

 guard-cell entails several advantages. In the first place it is obvious 

 that the whole mechanism will act with greater precision when two 

 longitudinal strips are " fixed," than it would do if the ventral wall 

 were only provided with a single median ridge. Further, the 

 interpolation of a thin strip between the two thickened ridges allows 



2f 



