RELATION OF AIR TO FOOD MANUFACTURE 79 



blade. The surface layer of the leaf is like a thin skin and 

 is called the epidermis. The green part of the leaf inside the 

 epidermis is the mesophyll. The epidermis is ordinarily as 

 thin as fine tissue paper and almost as transparent. It can be 

 quite easily peeled from some leaves, while on others it is too 

 delicate to be removed. The epidermis is found on petioles 

 and young stems, as well as on the blades. 



Although it is so thin the epidermis is able to protect 

 the mesophyll. If it is removed the lightest touch seems to 

 crush the mesophyll 

 and within a few min- 

 utes it will begin to 

 shrivel or turn black 

 from loss of water. If 

 the epidermis were not 

 present the mesophyll 

 would soon be de- 

 stroyed by the whip- 

 ping of the wind, as 

 well as completely 

 dried. 



86. Structure of the 

 epidermis. It is very 

 plain as you peel it off that the epidermis covers the entire 

 blade, and it is equally certain that air gets into the leaf in 

 some way. Just how it gets in is to be explained by examin- 

 ing the structure of the epidermis, and this must be done with 

 the microscope, for the unaided eye gives us little information 

 on this point (fig. 41). 



If a bit of the epidermis from a lily or a blue-flag leaf is 

 examined with a high-power microscope, it will be found that 

 it looks something like a brick pavement. It is made up of 

 many small oblong bodies closely joined together. These 

 bodies are like bricks only in shape and in the way they are 

 joined. They are called cells, and not only the epidermis but 



le. 



FIG. 41. Diagram of a leaf .section 



The view shows a part of a leaf, greatly magni- 

 fied. The lower surface and the cut edge are rep- 

 resented : u.e., upper epidermis; pal., palisade 

 tissue; sp., spongy tissue; le., lower epidermis; 

 tit., stoma 



