NUTRITION. 171 



structive and destructive changes go on at the same time 

 and without conflict. 



239. Respiration. — A series of katabolic changes is in- 

 cluded under the term respiration. It is a famih'ar fact tliat 

 the higlier animals cannot live without a constant supply of 

 oxygen and a corresponding excretion of carbon dioxide. 

 This is not so generally known to be true of plants. It is, 

 nevertheless, true that no plant can live without a constant 

 supply of oxygen and a corresponding excretion of carbon 

 dioxide. The processes by which oxygen is obtained and 

 carbon dioxide excreted constitute respiration. 



240. Respiratory ratio. — The ratio between the amount 

 of oxygen consumed and carbon dioxide produced varies 

 somewhat with the age and condition of the plant, as well as 

 with the circumstances under which respiration occurs. 

 Ordinarily the volume of carbon dioxide produced is approx- 

 imately ecjual to the volume of oxygen consumed, and the 



, , Cf), 



ratio may be expressed thus: — — -' = r. 



241. Respiration and photosyntax. — In the green plants 

 respiration is masked in daylight by photosyntax. When- 

 ever the green parts are sufficiently illuminated, the carbon 

 dioxide produced by their respiration is consumed in the 

 formation of carbohydrates for food. But when these parts 

 are not adequately illuminated, the process of photosyntax 

 is interrupted, and resjiiration can be studied. Tlie parts 

 of plants which are free from chlorophyll, such as young 

 flowers, buds, embryos, and the like, and all the non-green 

 plants, allow the respiratory changes to be demonstrated 

 readily. 



242. Aeration. — The oxygen consumed comes from the 

 atmosphere, or from the molecules of this gas dissolved in 

 water. Certain plants are adapted to aerial respiration, while 

 others are adapted to aquatic respiration, but in either case 



