RELATION OF AIR TO FOOD MANUFACTURE 81 



and includes a semiliquid, clear mass of living substance 

 (protoplasm) and a number of green granules called chloro- 

 plasts. The irregular cells near the lower epidermis have 

 large spaces between them and are called the spongy tissue. 

 Its many spaces give the air which enters through the lower 

 epidermis an opportunity to reach all the cells. 



The upper green cells are long, narrow, and cylindrical. 

 They stand side by side, "with the end of each cell toward 

 the epidermis, and are called the palisade layer. 



The green coloring matter, or chlorophyll, does not color 

 the cells green throughout, as one might suppose. In each cell 

 there are found some small, rounded grains that are bright 

 green in color, but the remainder of the cell is quite colorless. 

 The chlorophyll may be removed from the chloroplasts, leav- 

 ing the leaf white. These chloroplasts are much more num- 

 erous in the cells of the palisade than in those of the spongy 

 tissue, so that the upper side of a leaf is usually much darker 

 green than the lower side. It is upon these chloroplasts that 

 the light acts in the manufacture of sugar and starch. 



88. Entrance of materials into the leaf. Water, which 

 has been absorbed from the soil by the roots, passes up 

 the stem and reaches the various parts of the leaf through 

 the leaf veins. Those cells which are not in contact with the 

 veins receive water through other cells. The water supply 

 therefore reaches every cell of the leaf. 



Carbon dioxide, as well as other components of the air, 

 passes through the stomata by diffusion. The spaces between 

 the cells of the mesophyll allow the gas to spread through- 

 out the leaf. In this way carbon dioxide and water the 

 raw materials of food manufacture are brought together 

 in the cells of the mesophyll. 



89. Manufacture of carbohydrates. It is in the cells of 

 the mesophyll that a change occurs in the two compounds, 

 water and carbon dioxide, by which sugars are produced and 

 oxygen released. 



