MOVEMENTS OF STOMATA 103 



guard cell walls is so pronounced that the cell lumen is 

 much reduced. The guard cells contain abundant starch 

 grains ; this is the case even in leaves which do not other- 

 wise form starch, as in most monocotyledons. When the 

 guard cells are flaccid, because of low water content, they 

 are nearly straight and lie with their inner walls touching 

 each other, so that the stomatal pore is closed. With 

 increasing water content, and consequent greater turgor 

 pressure, the walls are expanded. As the outer wall — away 

 from the pore — is thinner it stretches much more than the 

 inner. In doing so it pulls the inner wall with it, so that 



Fig. 9. — Stoma of Cereal (diagrammatic), i. Open. 2. Closed. The 

 thickened portion of the guard-cell is shaded. 



the guard cell becomes bent outwards and the stomatal 

 pore is opened. That the opening of the stoma is actually 

 accompanied by a distension of the guard cells has been 

 shown by Schwendener (i 881), to whom much of our exact 

 knowledge of the mechanism of this movement is due ; 

 he found an increase in width of the guard cell of about 

 10 per cent. The effect of loss of water is strikingly shown 

 by plasmolysing a strip of epiderm with open stomata ; 

 closure at once takes place. 



Other types of structure and mechanism occur. The 

 most important of these, characteristic of the grasses and 

 cereals, may be described (Fig. 9). The two ends of the 

 guard cell are thin walled, and are joined on one side to those 

 of the sister guard cell. The wall of the middle portion is 



