DELATION OF AIR TO FOOD MANUFACTURE 71 



or epidermis, which covers it and the veins together. This 

 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 minutes it will begin to 

 shrivel or turn black from loss of water. It is easily under- 

 stood that if the epidermis were not present, the mesophyll 

 would soon be destroyed by the whipping of the wind, as well 

 as completely dried. The epidermis is therefore an important 

 protective structure. 



76. Structure of the epidermis. How is air able to enter the 

 leaf if the epidermis is a complete covering for the blade ? 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 

 examining the structure of the epidermis, and this must be 

 done with the microscope, for the unaided eye gives us little 

 information on this point. 



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 

 also all the other parts of the plant are made up of cells. The 

 cells differ very much from the bricks with which we com- 

 pared them. They are transparent and, so far as you can see 

 in the piece of epidermis, they are hollow. You will learn as 

 you examine cells more closely that they contain the living 

 material of the plant, though this is often so transparent that 

 it is difficult to see, as in this case. What you ordinarily see 



