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 the purpose of the petiole and green 

 blade (Fig. 3) or the significance of the various forms and 

 arrangements of leaves it will be necessary to examine the struc- 

 ture of the leaf and see the character of the apparatus that is 

 used in the performance of its work. The blades of the majority 



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

 ment of cells upon the upper side of the leaf. 



of leaves are flat. A 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 



