NUMBER OP STOMATES. 19 



astonishing in the supposition that molecules of moisture should pass 

 through interstices between the molecules of which the cell walls are 

 constructed ; and, with what is known of endosmosic and exosmosic 

 action, it seems to be not unreasonable to suppose that any moisture 

 passing through the cell wall from within outwards might, by the 

 increased density following the evaporation of a portion of it, deter- 

 mine a current to follow it which would flow continuously. To the 

 consideration of what function of the leaf is thus subserved we shall 

 afterwards return : it is with the arrangement for the passage into 

 the atmosphere of moisture taken up by the spongioles with which 

 alone we are at present concerned. 



Of the extent of the provision made for this evaporation some idea 

 may be formed from a consideration of the number of the stomata or 

 sto mates to be found in the leaves of plants, often symmetrically 

 disposed. The number varies in different plants, for which variation 

 a reason may be found in the different conditions of growth to which 

 they are subjected in their several natural habitats. In the back of 

 the leaf of the apple tree there are about twenty-four thousand 

 stomates to the square inch. In the leaf of the lilac there are 

 a hundred and sixty thousand of them to the square inch. 

 Sixty thousand have been reckoned in a square inch of the under 

 surface of the white lily and three thousand in a square inch of 

 the upper surface. In the leaves of the cherry-laurel there are 

 none on the upper surface of the leaf, but ninety thousand have 

 been counted on the lower surface of the leaf. In the true lilies 

 they are so large that they may be seen with the aid of a 

 simple lens of an inch focus. In the water lilies, and other 

 plants having leaves which float upon water, all the stomates 

 are on the upper surface, where alone evaporation can take place. 

 Leaves of plants which grow entirely under water, where there can be 

 no evaporation, have none. 



The mechanism of the opening and shutting of stomates has been 

 carefully investigated by Mohl, a distinguished German botanist, the 

 results are given in the Botanische Zeitung for 1866, p. 697 ; and it 

 appears that, while the general action is such as I have described, 

 other processes co-operate to ensure the result. It is partly controlled 

 by the effects of light, though depending mainly upon endosmose. 

 Mohl has shown that while the cells of the stomate themselves act so 

 as to open the stomate in moist weather and close it in dry, the 



