86 Agricultural Chemistry. 



compounds and employs oxygen in the process of respiration ana- 

 logous to that of animals. 



The magnitude of the former process can be realized when we 

 recall that a 12 ton crop of corn requires for its production four 

 tons of carbon dioxide. To secure this amount, the plants must 

 respire 10,000 tons of air or approximately one-fourth of the 

 total amount over an acre of land. 



The construction of organic compounds, which is a character- 

 istic function of the plant occurs principally in the leaf. It is 

 initiated by the green coloring matter known as chlorophyll. 

 This substance has been shown to be a specific but complex chem- 

 ical compound. It may be seen under the microscope as granules 

 clustered within the cells of all green plant tissues. In some 

 colorless fungi and lower plants it is lacking. Such plants do 

 not construct organic compounds independently but derive them 

 from previously existing vegetation. The green color of plants 

 is due to chlorophyll, as may be shown by extracting it with 

 alcohol. Such an extract is intense green in color, due to the 

 chlorophyll removed by the alcohol, while the extracted tissue is 

 bleached and colorless. In some unexplained manner this sen- 

 sitive compound, under the influence of light, induces the union 

 of carbon dioxide assimilated from the air, and water conveyed 

 from the root, with the production of the first carbohydrates of 

 the plant. 



It is not known whether this first product is starch, sugar, or 

 a simple precursor of these compounds. The process involves 

 the elimination of two parts of oxygen for each part of carbon 

 dioxide assimilated, as shown by the following general expres- 

 sion: Carbon dioxide + Water = Carbohydrate (Dextrose) 

 + Oxygen. 



The evolution of oxygen in this process has been proved by ex- 

 periments in which living leaves were confined in inverted jars 

 of water. A gas which collected above the water responded to 

 tests for oxygen and its volume was found to be equivalent to the 

 carbon dioxide taken up. The plant also performs through the 



