72 CUTICLE. STRUCTURE OF THE STOMATA. 



evaporating ; since, if they were to dry up, their vital properties 

 would be lost. Accordingly we find it absent in plants which 

 habitually live beneath the surface of the water, and from those 

 parts of others which are usually submerged ; whilst it is present 

 on those parts of the same plants, which are lifted into the air ; 

 as well as on all the soft parts of those, which are habitually and 

 entirely exposed to it. Its use is at once seen, when a portion of a 

 plant destitute of it is exposed to the air ; it then speedily dries 

 up and withers. On the other hand, the Oleander, exposed to 

 the intense sunshine of tropical Africa, maintains its verdure, 

 even in arid situations, by the great resistance to evaporation, 

 which its thick and almost leathery cuticle interposes. The best 

 mode of separating this cuticle, so as to become acquainted with 

 its remarkable firmness, is to soak a leaf for a few days, in water 

 rendered sour to the taste by a few drops of nitric acid ; and it 

 may then be easily stripped off. But its different layers can only 

 be seen by magnifying a very thin slice of the leaf cut across ; 

 so that its thickness, not its surface, is exposed to view. 



90. The whole of the softer portions of all plants growing in 

 air is covered by cuticle ; and in the young plant the entire 

 surface. It is only when the stem increases in diameter, and the 

 bark becomes hard and rugged, and occasionally scales off, that 

 the cuticle can no longer be distinguished. It is evident on 

 young shoots as on the leaves, and may be traced downwards to 

 the point of the root ; but this it does not cover. It also protects 

 all the organs of which the flower is composed ; but it is absent 

 at one point, for reasons hereafter to be stated (Chap, xii.) 

 The walls of the cells of the cuticle are often of considerable 

 thickness, giving firmness to the organ it covers: such is evident 

 in the leaves of the Holly, the hardened edges of which are com- 

 posed of cells continuous with those of the cuticle. 



91. The tissues protected by the cuticle are not entirely cut 

 off by it, however, from the external air ; for it has certain aper- 

 tures of a very peculiar character, which open or close under the 

 influence of light. These apertures are called Stomata (mouths). 

 They are usually of an oval form, and bounded by two kidney- 

 shaped cells containing green matter ; and it is by the expansion 



