84 FOOD ELABORATION AND RESPIRATION 



chlorophyll, the plant can not appropriate the carbon di- 

 oxid of the air. 



175. In most plants, this chlorophyll or leaf-green is 

 scattered throughout the green tissues in little oval bodies, 

 and these bodies are most abundant near the upper surface 

 of the leaf, where they secure a large amount of light. With- 

 out this green coloring matter, there would be no reason for 

 the large flat surfaces that leaves possess, and no reason for 

 the fact that the leaves are borne most abundantly at the 

 ends of the branches, where the light is most available. 

 Plants with colored leaves, as coleus, have chlorophyll, but 

 it is masked by other coloring matter. 'This other coloring 

 matter is usually soluble in hot water. Boil a coleus leaf and 

 notice that it becomes green and the water becomes colored. 



176. Plants grown in darkness are yellow and slender, 

 and do not reach maturity. Compare the potato sprouts 

 that have grown from a tuber lying in the dark cellar with 

 those that have grown normally in the bright light (Fig. 

 45). The shoots have elongated until the food which is 

 stored in the tuber is exhausted. These shoots have lived 

 useless lives. A plant that has been grown in darkness 

 from the seed will soon die, although for a time the little 

 seedling will grow very tall and slender. Light induces the 

 production of chlorophyll. Sometimes chlorophyll is found 

 in buds and seeds, but it is probable in most cases that 

 these places are not perfectly dark. Notice how potato tubers 

 develop chlorophyll, or become green, when exposed to light. 



177. Photosynthesis. — Carbon dioxid diffuses into the 

 leaf (173) and is used during sunlight, and oxygen is given 

 off. We have seen (172) that carbon dioxid will not support 

 animal life. Experiments show that carbon dioxid is absorbed 

 and that oxygen is given off by all green surfaces in the hours 

 of sunlight. How the carbon dioxid may be used in making 

 organic food is a complex question and need be considered 

 here only in a general way. 



