28 The Sugar-Beet in America 



taken up by plants, but while they may be used in various 

 plant processes, they are not essential to growth. 



These various soil compounds are also taken up by 

 osmosis, each one independent of the other. If the plant- 

 cells are low in one of the required substances that are 

 present in the soil solution, it passes through the cell- 

 wall of the root-hairs and from cell to cell to the place 

 where needed. The movement continues as long as the 

 compound is used by the plant if the supply in the soil 

 is maintained. If this supply becomes depleted, the 

 growth of the entire plant is retarded by a shortage of 

 this one element. This explains the importance of keep- 

 ing the soil well supplied with all the necessary plant- 

 foods. 



The processes taking place in the leaves are even more 

 interesting. The leaf is made up of layers of cells of 

 various kinds. On the surface of the leaf are tiny open- 

 ings called stomata through which air and other gases 

 pass freely. These stomata are much more numerous on 

 the under side of the leaf. The air, containing carbon- 

 dioxid gas, enters the leaf through the stomata and 

 circulates between the loose sponge cells, where a trans- 

 formation takes place. The cells of the leaf contain 

 chlorophyll, or leaf green, which, through the action of 

 sunlight, is able to cause a union of carbon dioxid and 

 water with the final formation of sugar. By this process 

 the greater part of the plant material is made. In this 

 laboratory the food of man and beast is prepared. If a 

 process similar to this did not take place in plants, it 

 would be only a short time till practically all animal and 

 plant life would disappear. 



