Plants in Relation to Man 515 



VII. THE MANUFACTURE OF FOOD BY PLANTS 



One of the most important differences between plants and 

 animals is that plants manufacture their own food from inor- 

 ganic substances, while animals do not. It is therefore upon 

 this manufacturing ability of plants that we depend for food, 

 directly when we live upon a vegetable diet and indirectly when 

 we live on meat and animal products. 



249. Sources of materials. The materials that plants 

 need for food manufacture are derived from the soil and from 

 the atmosphere. What is derived from the soil water has 

 already been considered. The minerals constituting the ash 

 remains of plants which have been burned are some of the 

 substances which have been absorbed through the roots and 

 assimilated into the plant tissue. But more than half the dry 

 weight of plants is made up of carbon. A small quantity of 

 carbon is absorbed by the root hairs from the decaying organic 

 matter in the soil, but the greater part of the supply is derived 

 from the carbon dioxid of the air. 



250. The work of the leaves : Carbohydrates. The organs 

 which take carbon dioxid from the air are the leaves, which are 

 green in all food-producing plants. The color is due to small 

 green bodies called chloroplasts present in the cells of the leaves. 

 The chloroplasts contain a green pigment called chlorophyll, 

 and green plants are often described as chlorophyll-bearing 

 plants. The chloroplasts are the manufacturing centers of 

 the leaves; they absorb the carbon dioxid, which is taken 

 from the air through very minute openings in the leaves, and 

 decompose it into its elements, carbon and oxygen. Oxygen 

 is released through the openings of the leaves to the air; and 

 the carbon is combined by the chloroplasts with hydrogen and 

 oxygen, obtained from the water of the sap, to form new com- 

 pounds called carbohydrates. It is supposed that sugars are 

 produced first and when a certain amount of sugar is accumu- 

 lated in the leaf cells, it is changed by the chloroplasts into 



