10 



HOW PLANTS GROW 



can be manufactured, except in sunlight, though it has been demon- 

 strated that strong artificial light will also produce growth. 



Assimilation. There are two general classes of products manu- 

 factured in the leaf, known as protein compounds and carbon- 

 hydrogen compounds. Protein compounds are all rich in nitrogen 

 but contain other elements as well. Carbohydrates do not contain 

 either nitrogen or minerals, but are compounds of hydrogen, oxygen, 



FIG. 3. Diagram illustrating the assimilation of food materials by a plant. The water 

 containing minerals and nitrogen passes upward into the leaves, where it unites with carbon 

 and oxygen. The elaborated plant food then passes to all parts of the plant. The upward 

 movement of water and return flow of plant food are through different channels. B is ar 

 enlarged section of leaf taken at point A. Note the opening for air at S. C is a single 

 plant cell from the leaf. The dark spots are chlorophyll bodies. 



and carbon, such as starch and sugar (Fig. 3). Fats also contain 

 only hydrogen, oxygen, and carbon, but in a more concentrated 

 form. Fats and oils are about 2*/ times as valuable as starch or 

 sugar for feeding stock. 



Distribution of Manufactured Products. When the protein 

 and carbohydrate compounds have been manufactured, they must be 

 redistributed through all parts of the plant and some must be re- 

 turned back to the roots. The return flow takes place through a 

 set of vessels similar to the system which carried the sap upward. 



