THE LEAF 171 
186. 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, 
under 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 (CO,) is combined with water 
(H,0O) 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 O, 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 (H:0) + 6(COz) = (CseHi00s) + 6(02) = 12(0). 
