80 ELEMENTS OF GENERAL SCIENCE 



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

 The cells differ very much from the bricks with which we 

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

 see in the piece of epidermis, they are hollow. The cells con- 

 tain the living material of the plant, though this is often so 

 transparent that it is difficult to see. Ordinarily we see only 

 the wall which incloses the living part of the cell. 



A few of the cells of the epidermis 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 together tightly, as the surfaces 

 of all the other cells do, and this leaves a little space between 

 them which is an opening through the epidermis. 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. 



If you look in the microscope the stomata seem to be far 

 apart, but one must remember that the view in the instru- 

 ment 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 

 sentence. 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 and much water vapor to escape. 



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. 



87: Structure of the leaf interior. The interior, or meso- 

 phyll, of the leaf (fig. 41), like all the other parts of a plant, 

 is made up of cells. Each cell has a thin, transparent wall 



