172 Chapters in Modern Botany chap. 



The surface is often stiff and firm, and its outermost 

 coating thick. We recognise the use of this as a protection 

 to the delicate Hving parts within. When there is a waxy 

 varnish or a covering of hairs the protection from cold and 

 from loss of water will be all the greater. In many cases 

 the outermost layer, especially on the upper surface, consists 

 of units or cells with thick corky upper walls (the so-called 

 cuticle)^ and we know how impermeable cork is to the 

 passage of water. But in many cases the cells of this 

 colourless layer of epidermis are very rich in water ; they 

 form a layer of little water-bags all over the leaf. We can 

 see the use of these ; they lie between the outer world and 

 the more delicate living cells in the heart of the leaf, so that 

 when a rapid loss of water occurs from the surface, it is not 

 the internal parts which directly suffer, but the hardier and 

 less intensely living epidermis. 



In many cases the leaf is so well protected by its hard 

 external sheath that there can be very little transpiration 

 except by the regular apertures — the stomata. These are 

 exceedingly numerous, usually several hundred on a square 

 inch. In most leaves they are virtually restricted to the 

 under surface, but in those of plants like the water-lilies, 

 which float on the water, they occur where alone they can 

 be of use — on the upper surface, and in leaves which grow 

 upright they are equally numerous on both sides. 



It is necessary to examine more carefully into the nature 

 of these stomata. Each aperture is bounded by two special 

 cells of the epidermis, called guard-cells. Unlike most of 

 the cells of the epidermis, these are green, and therefore 

 able to form starch like the tissue in the heart of the leaf. 

 Furthermore, they are so disposed that when they are rich 

 in water — in other words, when they are swollen, " tur- 

 gescent " — the stoma or aperture between them is opened ; 

 but when the guard-cells contain for the time being less 



