94 VEGETABLE PHYSIOLOGY 



curved so that the orifice is widely open. This is helped 

 by the thickening of the free edges, which makes it difficult 

 for them to swell in the direction of each other. When, 

 on the other hand, they lose their water, they relax, and 

 their edges coming into contact, the aperture between them 

 is more or less completely closed (fig. 66). 



The number of the stomata varies very considerably. 

 The following table will give some idea of their abundance 

 in leaves, and it will be observed that the number of stomata 

 is usually greatest in those leaves from whose upper surface 

 they are entirely absent. 



Stomata in one Square Inch of Surface 



The modification of the turgescence of the guard-cells 

 is caused by the osmotic transference of water between 

 them and the other cells of the epidermis from which they 

 are separated by thin walls. The vapour which is in the 

 intercellular space below them does not penetrate them, 

 the walls abutting on the space being thick and cuti- 

 cularised. The osmosis alluded to may be associated with 

 the presence of the chloroplasts in the guard-cells, which 

 are instrumental in the production there of various sub- 

 stances, so that their contents have a higher osmotic 

 equivalent than those of the epidermal cells which are con- 



