CHAPTER XIX. 

 RESPIRATION. 



Ordinary Eespiration in the presence of free oxygen of the 

 atmosphere : aerobic respiration. — One of the most familiar 

 physiological processes carried on by living animals is that of 

 respiration, during which there is a constant interchange of gases 

 between the body of the animal and the surrounding air : the 

 oxygen of the air is inspired into the lungs, and from the latter 

 carbon dioxide gas is breathed out into the atmosphere. So 

 long as life exists respiration goes on continuously, and one of 

 the certain signs of death is the cessation of the process. 



Respiration, however, is not confined to animals, but is 

 carried on by all ordinary plants, and is as necessary for 

 their existence as for the existence of animals. 



The amount and rapidity of respiration is usually much greater 

 in animals than in plants, but the process is essentially the same 

 in both classes of organisms. It is well known that animals die 

 when the supply of fresh air is cut off, and plants soon show 

 signs of ill-health under similar conditions. In ordinary farm 

 and garden practice the parts above ground always obtain 

 sufficient oxygen for all their requirements, but the roots of 

 plants are often seriously injured through want of a suitable 

 supply of fresh air in the soil. The unhealthy appearance of 

 over-watered pot plants and of crops growing in badly-drained 

 ground is primarily due to an insufficient supply of oxygen to 

 their roots. Seeds buried too deeply do not obtain sufficient 

 fresh air for normal respiration and either do not germinate at 

 all or do so in an unsatisfactory manner. 



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