X] OPENING AND CLOSING OF STOMATA 103 



in apposition on a sort of hinge to the rigid roofs of 

 their neighbours ; hence any movement of the lower and 

 lateral thin floors of the neighbouring cells caused by 

 their distension or collapse, must cause the guard-cells to 

 approach or recede from one another, and so narrow or 

 widen the slit-like pore. Similarly, any distension or 

 collapse of the guard-cells themselves must tend to drive 

 apart, or to approximate, their rigid floor and roof, and so 

 straighten vertically or bulge forward to meet one another, 

 the thin walls bounding the aperture. Yet, again, any 

 distension of the curved sausage-shaped guard-cells, with 

 their mutually affixed ends, must tend to make their 

 concave lateral walls recede from or approach one another ; 

 and it is obvious that in the mobility due to the exercise 

 of these complex movements we have efficient mecha- 

 nisms for the opening and closing of the stomata, though 

 it is extremely difficult to follow the movements step by 

 step in any given case. 



Let us suppose, for instance, that the two guard-cells 

 of a typical stoma are in a state of equilibrium such that 

 their thick and rigid floor and roof are approximated 

 elastically, and their thin lateral walls are not distended : 

 the thin walls bounding the pore are then bulging towards 

 and meeting one another, while the walls contiguous with 

 the neighbouring cells are nearly straight in the vertical 

 plane. Now suppose an influx of water into the guard- 

 cells to occur and to distend them. Neglecting any other 

 possible factors, the effect of the distension will be to drive 

 the rigid floor and roof of the guard- cells further apart, 

 and tend to straighten vertically the thin bulging walls 

 bounding the aperture, i.e. to open the stoma. 



Researches have made it probable that the opening of 

 stomata in bright sunlight is really due to the formation, 

 in the chlorophyll-corpuscles of the guard-cells, of osmotic 



