40 MANUFACTURE OF FOODS AND FUELS 



(CO2) and water (H2O) to form sugar (CeHijOe) is ac- 

 complished only by means of light energy absorbed by 

 the green parts of plants and is therefore called photo- 

 synthesis. It is the most important chemical reaction 

 in nature, for it underlies the manufacture of all foods 

 and fuels. The rays absorbed are mainly near the red 

 and the blue in the spectrum; the absorbing agents are 

 green pigments in cells near the surface of the bodies 

 of plants. 



Chlorophyll. — The green color of leaves or other 

 green parts of plants is due to the presence of certain 

 substances, the chlorophyll pigments, or chlorophyll, con- 

 fined in most instances to small bodies called plastids or 

 chromatophores. Chlorophyll absorbs light of certain 

 colors or wave-lengths only. The colors which pass 

 through give the characteristic green to leaves and other 

 chlorophyll-containing parts or organs. ChlorophyU, 

 soluble in alcohol, chloroform, ether, and other fat sol- 

 vents, appears to be composed of four compounds of 

 distinct color qualities. These have been separated with 

 great skill, and their composition determined. Two of 

 them are green, two are yellow. They are crystalline 

 substances, complex and unstable, though not equally so, 

 as shown by the fact that the yellowing of leaves is due 

 to the persistence of one or both of the yellow pigments 

 while the green ones are undergoing withdrawal or 

 decomposition. 



Occurrence of Chlorophyll. — The chlorophyll pig- 

 ments do not occur in all plants, and they occur only or 

 mainly in those parts of green plants which are exposed 

 to light. The pigments occur only in the plastids or 

 chromatophores of cells situated so near the surface of 

 the plant body that an adequate supply of light will 

 penetrate them. Chlorophyll will be formed, however, 

 only under suitable conrhtions, namely, when the plant 

 absorbs suitable and sufficient iron salts from the soil and 

 there is enough light. Although iron is not a constituent 



