68 ELEMENTS OF AGRICULTURE 



the intercellular spaces, it is still outside the cells where 

 the food is manufactured. It passes through the cell- 

 walls by osmosis. Within the leaf cells the carbon is re- 

 moved and united with the nutrients brought up by the 

 roots to form starch and other plant foods. The surplus 

 oxygen passes back into the intercellular spaces by osmosis, 

 and thence through the breathing pores to the air. 



THE MANUFACTURE OF FOOD MATERIALS 



66. Carbohydrates. The most abundant food material 

 that is built up by the plant is starch. This is a compound 

 of carbon, hydrogen and oxygen (C 6 H 10 O 5 ). Starch is 

 formed only in sunlight, and then only by green plants. 

 The chlorophyll plays an important part in its manufacture. 



Starch is insoluble, and hence cannot move through the 

 cell walls. But the plant can readily change it to sugar 

 and other soluble substances, so that it can be transferred. 

 It can then be reconverted into starch. These changes 

 are independent of light. The starch formed in potato 

 leaves can be changed to sugar, transferred to the tubers 

 below ground, and there reconverted into starch. When these 

 tubers sprout the next year, the starch is again changed 

 to soluble compounds. All these compounds are carbo- 

 hydrates. Chemically, they are distinguished from other 

 compounds of carbon, hydrogen and oxygen, in that they 

 contain the hydrogen and oxygen atoms in the proportion 

 of two to one, that is the proportion of water (H 2 0). 

 This is why they are called hydrates. Some of the other 

 important foods that plants form are fats and protein. 



67. Fats. The fats differ from starch and sugar iu hav- 



