BOOK IV 



CHAPTER XXX 

 RESPIRATION 



ONE of the moat remarkable properties of living substance is its 

 dependence on a supply of oxygen. The vital processes in the cell 

 substance by which energy is set free and food obtained depend on 

 this, and carbon dioxide, the chief end product of these processes, 

 must be got rid of, as well as a sufficiency of oxygen obtained. Many 

 kinds of bacteria e.g., tetanus bacilli and parasitic worms, which 

 live in the intestine, are what are termed " obligate anaerobes." They 

 secure their oxygen by decomposition of foodstuffs, and are poisoned 

 by free oxygen. All other bacteria, the higher plants, and animals, 

 are aerobes, and take up free oxygen from the atmosphere by a 

 respiratory process. Plants both excrete carbon dioxide and assimi- 

 late it. 



By the aid of chlorophyll energized by sunlight, plants synthesize 

 water and carbon dioxide into formic aldehyde, which by molecular 

 condensation is converted into an aldoae; hence arise sugar and starch. 

 At the same time as this remarkable synthesis goes on, the plant 

 protoplasm uses up oxygen and produces carbon dioxide. From the 

 nitrates and other salts absorbed by the rootlets and the aldose syn- 

 thssized in the leaves the plant proteins are built. Sunlight forwards 

 this synthesis, converting nitrates in watery solution into nitrites, 

 an endDtharmic reaction; ths sugars and starch may also be converted 

 into fat. The source of the energy of all these syntheses in the plant 

 is the sun's rays. The chlorophyll absorbs the light rays, and trans- 

 forms them into energy which is stored as potential energy in the 

 proteins, carbohydrates, and fats, built by the plant. These form 

 the foodstuffs of bacteria, moulds, and animals, by which they are 

 broken down again into water, carbon dioxide, salts, and simple com- 

 pounds of nitrogen the materials for fresh plant synthesis. So the 

 cycle of life proceeds (Fig. 130). The bacteria form the final link in 

 the chain; some decompose dead animal and plant matter (denitrify- 

 ing bacteria), while other symbiotic bacteria help the rootlets of certain 

 plants (legumes) to secure atmospheric nitrogen (nitrifying bacteria). 

 If it were not for bacteria, the world would quickly become cumbered 

 up as a charnel house. The circulation of nitrogen in nature is shown 

 in Fig. 131. 



It has recently been found that when carbon dioxide gas is passed 



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