RELATION OF AIR TO FOOD MANUFACTURE 73 



cells. Each of these cells consists of a thin, transparent watt, 

 a semi-liquid, clear mass of living substance (protoplasm) fill- 

 ing the space within the wall, and a number of green granules 

 called chloroplasts. There are several other important things 

 in the cell, but we need not concern ourselves about them at 

 present. The cells in the part of the leaf near the lower epi- 

 dermis are arranged very irregularly, with large spaces between 

 them. This part of the mesophyll is called the spongy tissue^ 

 and its many spaces give the air which enters through the 

 lower epidermis an opportunity to reach all the cells of this 

 tissue. Since there are 

 few stomata or none 

 at all in the upper epi- 

 dermis, little air enters 

 through it. 



The mesophyll of 

 the upper surface is 

 made up of cells of 

 fairly regular shape. 



They are long, narrow, FlG - 39 - Diaram of a leaf section 



and cylindrical. They 

 stand side by side, with 

 the end of each cell 

 toward the epidermis, 

 and are fitted together quite closely. They may be arranged 

 in one or more rows, the surface row being immediately 

 under the upper epidermis. There are narrow spaces be- 

 tween them, so that the air which has entered the spaces in 

 the spongy tissue is able to penetrate into this more compact 

 layer. It is called the palisade layer. 



The green coloring matter, or chlorophyll, is distributed in 

 a very peculiar way. It does not color the cells green through- 

 out, as one might suppose. In each cell there are found some 

 small, rounded grains that are bright green in color, and the 

 remainder of the cell is quite colorless. These grains are 



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; I.e., lower epidermis; 

 St., stoma 



