THE LEAF 171 



i86. Photosynthesis. — In our examination of the internal 

 structure of the leaf, the chlorophyll bodies (184) were found 

 to contain small granules of starch which the chlorophyll, 

 imder the stimulus of light, had elaborated as a nutriment for 

 the plant tissues. Hence, the leaf may be regarded as a 

 factory in which vegetable food, mainly starch, is manufac- 

 tured out of the water brought up from the soil, and the carbon 

 dioxide derived through the stomata from the atmosphere. 

 In this process carbon dioxide (COg) is combined with water 

 (HgO) in such proportions that part of the oxygen is returned 

 to the surrounding air. This is a fundamental food-forming 

 process characteristic of green plants, and can take place 

 only in the light. For this reason it has been named Photo- 

 synthesis, a word which means " building up by means of 

 light," just as photography means " drawing or engraving 

 by means of light." 



In carrying on the operation of photosynthesis, sunshine 

 is the power, the chlorophyll bodies the working machinery, 

 carbon dioxide and water the raw materials, and starch or oil 

 the finished product, while oxygen and the water of trans- 

 piration represent the waste or by-products. 



187. How the new combination is effected. — It may 

 seem strange that a gas and a liquid should combine to make 

 something so different from either as starch, but their chemi- 

 cal constituents are the same in different proportions. Water 

 is made up of 2 parts hydrogen and 1 part oxygen; carbon 

 dioxide, of 1 part carbon and 2 parts oxygen, while starch 

 contains carbon, hydrogen, and oxygen, in the ratios of 6, 

 10, and 5, respectively. Hence, by taking sufficient quanti- 

 ties of water and carbon dioxide and combining them in the 

 proper proportions, the leaf factory can turn them into 

 starch. If we use the letters C, H, and 0, to represent Car- 

 bon, Hydrogen, and Oxygen, respectively, the new combina- 

 tion of materials can be expressed by an equation; thus: — 



water carbon dioxide starch by-products 



5(H20) + 6(C02) = (CeHioOe) + 6(02) = 12(0). 



