94 PLANT BIOLOGY 



Chlorophyll ("leaf green") is the agent that secures 

 the energy by means of which carbon dioxid is utilized. 

 This material is contained in the leaf cells in the form of 

 grains (p. 86); the grains themselves are protoplasm, only 

 the coloring matter being chlorophyll. The chlorophyll 

 bodies or grains arc often most abundant near the upper 

 surface of the leaf, where they can secure the greatest 

 amount of light. Without this green coloring matter, 

 there would be no reason for the large flat surfaces which 

 the leaves possess, and no reason for the fact that the 

 leaves are borne most abundantly at the ends of 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. 



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. 

 The shoots have become slender and are devoid of chloro- 

 phyll ; and when the food that is stored in the tuber is 

 exhausted, these shoots will 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: why? Light favors the production 

 of chlorophyll, and the chlorophyll is the agent in the mak- 

 ing of the organic carbon compounds. Sometimes chloro- 

 phyll is found in buds and seeds, but in most cases these 

 places are not perfectly dark. Notice how potato tubers de- 

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



Photosynthesis. — Carbon dioxid diffuses into the leaf ; 

 during sunlight it is used, and oxygen is given off. How the 



