70 BOTANY 



material called cutin. These external layers can be 

 stripped off from large pieces of the surface in a kind of 

 pellicle, which is known as the cuticle. It is developed 

 more freely over the leaves than over the stem. 



This layer serves too as a protection against cold. 

 For this purpose many plants have an additional safe- 

 guard, in the shape of hairs, or outgrowths of the 

 epidermal cells, forming a fine felt work over their sur- 

 faces, clothing them indeed in a kind of cotton garment. 



Both cuticle and hairy coating serve also to protect 

 the delicate surface from injury by rain. 



The outer coating or cuticle, covering as it does the 

 whqle exterior, would be a source of danger to the plant 

 by preventing almost all evaporation, if it were altogether 

 intact. The epidermis is pierced by small apertures, 

 which are the openings of the system of intercellular 

 spaces or passages we saw to be developed in the root 

 and which we now find to extend throughout the whole 

 of the shoot as well. These stomata, as they are called, 

 are more numerous in the leaf than in the stem, but they 

 are present in the latter so long as it is young. The 

 aperture or stoma is surrounded by two cells called 

 guard-cells, which are attached together at their ends 

 but not in their centre. They are kidney-shaped in 

 appearance, and when filled with water they stretch so 

 as to draw apart in the centre, opening the stoma (Fig. 

 28). When the water is withdrawn from them they fall 

 together and close the aperture. This arrangement 

 thus allows the necessary evaporation of water to take 

 place. The vapour is formed in the intercellular 

 passages and passes out through the stomata, the 

 width of the apertures being regulated by the amount of 

 water in the guard-cells, which in turn depends on the 

 amount of water in the plant. 



The layer of cells between the central cylinder and 

 the epidermis, which is the continuation backwards of 



