TKANSPIKATION 



in contact with the other epidermal cells, however, re- 

 mains thin. When the guard-cells are full of water, 

 their form and mode of attachment cause them to become 

 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 edges of the guard-cells when viewed in section are 

 slightly convex (fig. 67). The turgor which results from 

 the imbibition of water stretches them in the vertical direc- 

 tion as well as in the horizontal one. This tends to lessen 

 the vertical convexity, and at the same time to cause a 

 considerable vertical tension. When the escape of water 

 relieves this tension the thickened upper corners of the 

 cell recoil, lessening the vertical diameter of each and 

 increasing this convexity, sometimes bringing the con- 

 vexities of the two cells into contact with each other, and 

 so completely closing the aperture. 



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 



Mezereon 

 Pseony 

 Vine . 

 Olive . 

 Holly 

 Laurustinus 

 Cherry-laurel 

 Lilac . 

 Hydrangea 

 Mistletoe 

 Tradescantia 

 House -leek 

 Garden Flag 

 Aloe . 

 Yucca 

 Clove Pink 



