THE EPIDEBMIS AND ITS APPENDAGES. 21 



hollow chambers or air-cavities in the lower stratum of paren- 

 chyma. The aqueous contents of the cells of the parenchyma, 

 throughout the whole interior of the leaf, are thus brought into 

 immediate contact with the external air, whilst at the same 

 time the evaporation of their contents is controlled and regu- 

 lated by these foliar apertures. 



This is done in the following manner. The slit or perfora- 

 tion in the epidermal surface lies between two cells, which, 

 unlike the rest of the cuticular cells, generally contain some 

 chlorophyll, and in this respect resemble the parenchyma 

 beneath. These cells are exceedingly hygrometrical, or affected 

 by moisture. When the atmosphere is damp, these two cells 

 become swollen and turgid, and by their curvature outwardly, 

 open the orifice and allow the free escape of the superfluous 

 water ; but when the atmosphere is dry, they straighten and 

 lie parallel, their sides being brought into close contact, thus 

 closing the aperture and stopping evaporation the moment 

 it becomes injurious to the plant. The stomata or pores of 

 plants are therefore analogous to the governor in machinery, 

 and are clearly designed to regulate the operation of the 

 vegetable mechanism, and to promote the healthy passage of 

 fluids through the system. 



The structure of the stomata or pores of plants, may be 

 readily perceived on the epidermis of the lily, (Pig. 3,) where 

 they are unusually large. The epidermis must be carefully 

 removed, and having been freed from all its chlorophyll, or 

 green matter, it must be placed between two strips of glass 

 with a drop of water between them so as to give it the neces- 

 sary degree of transparency. Water ought, for this reason 

 always to be used whenever objects selected from the tissues 

 of vegetables are examined microscopically. The epidermis 



3 



