Chemical Nature of Constituents 99 



vegetation of the whole eartli (allowing for 25 per cent of the earth 

 surface as being nnproducti\c), coxering 10,160 million hectares, 

 will require an annual consumption of 90 billion kg of COo. The 

 whole atmosphere contains 2,100 billion kg of CO^, thus allowing 

 only for about 25 annual crops, a xery small figure indeed. Others 

 have calculated, howe\er, that the green plants consume annually 

 only one-se\ enteenth of the CO:., of the air in 1 year, which amounts 

 to 30 billion kg annually. Ebermeyer further reported that, out of 

 the 3,000 kg of carbon synthesized by 1 hectare of forest, 1,491-1,792 

 kg was con\erted to wood and 1,196-1,467 kg to litter. The latter 

 is returned immediately to the soil and becomes subject at once to 

 decomposition by microorganisms. 



Chemical Nature of Plant and Animal Constituents 



The plant and animal bodies are made up of numerous organic 

 compounds. Attention will be directed to only the more important 

 and more abundant substances, the decomposition of which by 

 microorganisms has been studied in greater detail and contributes to 

 oiu- knowledge of the cycle of life in the soil: 



1. Fats, oils, waxes, sterols, and terpenes. 



2. Carbohydrates, including the simple sugars or the mono-, di-, 

 and tri-saccharides, the starches, the hemicelluloses (comprising the 

 pentosans and hexosans ) , the polyuronides ( pectins, gums, and muci- 

 lages), and true cellulose. 



3. Organic acids, including saturated fatty acids, oxy-fatty acids, 

 and unsaturated acids. 



4. Aldehydes, ketones, and alcohols, including aliphatic, poly- 

 valent, and unsaturated alcohols. 



5. Lignins, compounds which are frequently spoken of as "in- 

 crustants." They are believed to form definite chemical or physical 

 compounds with the celluloses. Some believe, however, that cellu- 

 lose and other carbohydrates do not form any chemical compounds 

 with lignins and may not e\en form any homogeneous mixtiu'cs. 

 This concept is substantiated by the fact that the lignin content of 

 plants varies considerably, depending on the plant and on the stage 

 of growth, and may even vary in the different tissues of the same 

 plant. 



6. Cyclic compounds, including hydrocarbons, phenols, quinones, 

 tannins. 



