CUTICLE AND STOMATA. 17 



These slides are very useful to those persons who live 

 in cities, or who have not yet studied plant-life for them- 

 selves ; and I doubt not that they will lead many a careless 

 eye to look for other examples, and to find an endless 

 variety in the garden and the field. 



These pores, called stomata, are absolutely necessary to 

 vegetable life. Leaves are the organs of respiration the 

 lungs of a tree, and the stomach also ; for they send back 

 nutrition to the trunk and stem, take up the sap which 

 rises from the root, give it the needful quantity of carbon, 

 expose it to the action of the air, and cause the super- 

 abundant moisture to evaporate. All this is done by the 

 agency of the little dots we call stomata. And this is the 

 way in which they act : We see that the cuticle is formed 

 of a single layer of cells ; these contain air and not fluid, 

 as do the cells of the pulp or parenchyma; also they are 

 so closely fitted to each other as to confine that moisture 

 which otherwise would be too quickly evaporated by a hot 

 sun, and the leaf soon dried up and withered ; but at the 

 same time, as air is necessary to the inner cells of a leaf 

 or flower, these stomata, or openings, are placed in great 

 numbers in the cuticle, acting like valves, which admit air 

 freely, give out surplus fluid, and take in atmospheric 

 moisture when required. They are bordered by cells of 

 peculiar form, usually kidney-shaped, with an oval aperture 

 in the centre; and these "guard cells " dilate and con- 

 tract, closing or opening the passage according to the 

 necessities of the plant. On a hot day they will close, to 

 defend the inner cells from exhausting heat : in dry weather, 

 when the stem does not give enough fluid for the nourish- 

 ment of the leaves, then the stomata open at night and 

 drink in the night- dew, but close again as soon as the 

 cavities of the leaf are full. The number of pores in a 

 square inch of surface is amazing; e.g. we find that a 

 square inch of the leaf of 



Hydrangea contains . . .160,000 under surface 

 Iris ... 12,000 both surfaces 



Houseleek . . . 10,710 upper surface 



2 



