RESPIRATION AND FOOD PRODUCTION 23 



organs, leaves are the laboratories in which plant food 

 is manufactured out of the crude materials brought up 

 from the soil by the sap, and those absorbed through the 

 stomata from the gases of the atmosphere. Carbon di- 

 oxide taken from the atmosphere is somehow used up in 

 this operation, and the oxygen, which is not needed by 

 the plant, is given back to be consumed by animals. This 

 is the most important work the leaf has to do, and because 

 it can take place only in the light, has been named by 

 botanists Photosynthesis^ a word which means " building 

 up by means of light," just as photography means "drawing 

 or engraving by means of light." 



25. Why Leaves are Green. Has the color of the 

 leaf anything to do with this function ? It will help to a 

 correct answer if we remember that herbs grown in the 

 dark, and parasites like the dodder and beech drops 

 (Epiphegus\ which steal their food ready made from 

 the tissues of other plants, and so have no need to manu- 

 facture it for themselves, always lose their green color. 

 Place a seedling of oats or other rapidly growing shoot in 

 the dark for a few days and note its loss of color. Leave 

 it in the dark indefinitely, and it will lose all color and die. 

 Hence we may conclude that there is some intimate con- 

 nection between the action of light and the green coloring 

 matter of leaves. This green matter is called Chlorophyll, 

 a word meaning " leaf green," and physiologists tell us 

 that through its agency the crude substances brought up 

 from the soil in the sap and the carbon dioxide of the air 

 are converted into nourishment. 



26. Starch as Plant Food. It is the office of chloro- 

 phyll to manufacture a particular class of plant foods 

 known as carbohydrates. The commonest and most impor- 

 tant of these is starch, the presence of which can generally 

 be detected without much difficulty. Boil a few leaves of 

 bean or sunflower, tropaeolum, etc., for about fifteen min- 

 utes, and soak them in alcohol until all the chlorophyll is 

 dissolved out. Rinse them in water, and soak the leaves 



