8 THE WORK OF THE LEAF 



more important work performed by the leaves is the construction 

 of foods, the giving off of water or transpiration, and breathing 

 or respiration. The magnitude of this work far exceeds the 

 energy expended in all the industries of the world. The leaves, 

 however, accomplish this work so quietly and economically that 

 most people are scarcely conscious of it. 



In order to understand why the leaf has a petiole and green 

 blade (Fig. 3) or why it assumes various forms and arrangements 

 it will be necessary to examine the structure of the leaf and see 

 the character of the apparatus that is used in the performance of 

 its work. The blades of the majority of leaves are flat. A 



FIG. 4. Blade of lilac leaf cut across showing the more compact arrange- 

 ment of cells upon the upper side of the leaf. 



section through such a blade, cut as in Fig. 4 so that we can look 

 into the end of the blade, shows that the leaf is composed of a 

 complicated arrangement of cells. Fig. 5 is a greatly enlarged 

 view of Fig. 4 taken at X. 



4. The Epidermis. It is now seen that a layer of compact cells 

 (Fig. 5, e) surrounds the leaf on all sides. In fact such a layer 

 covers all parts of the plant body. This layer of cells, the epi- 

 dermis, is provided with minute openings or stomata (sing, 

 stoma), Fig. 5, s. The stomata are especially abundant in the 

 epidermis on the under surface of the leaf and often quite lacking 

 from the upper surface. A better idea of these openings and of 

 the epidermal cells may be gained by stripping off the epidermis 



