72 ELEMENTS OF GENERAL SCIENCE 



in the piece of epidermis is really nothing but the wall which 

 incloses the living part of the cell. 



The cells of the epidermis are not all alike ; a few of them 

 are crescent-shaped or bean-shaped instead of rectangular. 

 These cells are always in pairs, with their concave surfaces 

 next to each other. These curved surfaces do not come to- 

 gether tightly, as the surfaces of all the other cells do, and 

 this leaves a little space between them which makes an open- 

 ing through the epidermis. This explains how the water gets 

 out. There are many such openings in the epidermis. Of 

 course, each opening is very small, but the great number makes 

 up for the small size. These openings are called stomata, and 

 the two bean-shaped cells with each stoma are called the guard 

 cells. The guard cells make the opening larger or smaller by 

 changing their shape, but it is not entirely clear what service 

 is actually done to the plant by this change. 



The number of stomata is much greater than at first 

 appears. If you look in the microscope, they seem to be far 

 apart, but one must remember that the view in the instrument 

 is very much magnified. Perhaps half a dozen stomata are 

 seen, apparently much scattered over a wide surface, but in 

 reality the surface of the epidermis seen under a high-power 

 microscope is not larger than the period at the end of a sen- 

 tence. It is common for leaves to have more than 60,000 

 stomata to the square inch, and some leaves have as many 

 as 700,000 per square inch. Most of these are found on the 

 lower surface. It has been estimated that one large sunflower 

 leaf contains 13,000,000 stomata. This great number allows 

 the carbon dioxide to enter freely. 



The cells of the epidermis are not always so regular in 

 shape as in the lily. In very many plants they are extremely 

 irregular, though always fitting together very closely. The 

 stomata, too, vary in shape and appearance. 



77. Structure of the leaf interior. The mesophyll of the 

 leaf (fig. 39), like all other parts of a plant, is made up of 



