192 ELEMENTARY STUDIES IN BOTANY 



sections through some relatively thick, spongy leaf, like that 

 of hyacinth or of lily, should be made. In such sections 

 even a low power of the microscope will show three distinct 

 regions (Fig. 157). 



(1) Epidermis. Bounding each side of the leaf is a layer 

 of cells fitting closely together and usually without chloro- 

 plasts (Fig. 157). This is the epidermis, which is like a layer 

 of water-proof material preventing excessive loss of water 

 from the working cells within. The resistance to the escape 

 of water is much increased by a substance formed on the outer 

 walls of the epidermal cells, known as cutin, which forms a 

 covering of the epidermis called cuticle. This layer of 



FIG. 158. Section through a small portion of a yew leaf, showing the epidermis (e) 

 with its thick cuticle (c), and the upper part of the palisade layer (p). 



cuticle makes the outer epidermal walls look thick, and the 

 thicker it is the more resistant is the leaf to the loss of water 

 (Fig. 158). In plants of dry regions the cuticle may become 

 excessively thick. An epidermis overlaid by cuticle forms 

 not only a water-tight layer, but also an air-tight one. It is 

 evident that the cells at work in food manufacture cannot 

 be shut off from the air, for there must be an intake of carbon 

 dioxide and an outgo of oxygen. For this reason, there is 

 developed in the epidermis a set of guarded openings, the 

 stomata (singular stoma). 



. The general outline of a stoma may be seen by peeling 

 off the epidermis and examining it in surface view rather 

 than in section (Fig. 159). In surface view it appears as 

 two crescentic epidermal cells forming between them a slit- 



