234 RESPIRATION 



Each living cell of the body of a plant respires, the oxygen 

 necessary for the process being supplied from the air which 

 penetrates through the stomata and lenticels and permeates 

 throughout the plant in the intercellular spaces. 



In all the higher plants the products of respiration under 

 normal conditions are carbon dioxide gas and water. As the 

 carbon of the carbon dioxide is derived from the compounds 

 within the body of the plant, it is clear that the process is a 

 destructive one and must result in a decrease in the dry weight 

 of the plant. The Seedlings of cereals and many other plants 

 when allowed to grow in the dark often lose about half their dry 

 substance in two or three weeks. 



In this respect respiration is essentially the opposite of the 

 ' assimilation ' process in which there is a fixation of carbon and 

 a consequent increase in dry weight of the plant. Moreover, 

 respiration goes on in all living cells, both in darkness and in 

 Ught, whereas ' carbon-fixation ' is only carried on by those cells 

 which contain chloroplasts, and in these only when they are 

 exposed to light. 



During respiration oxygen is consumed and carbon dioxide is 

 set free into the air, but in green plants exposed to daylight the 

 ' carbon-fixation ' process consumes twenty or thirty times as 

 much carbon dioxide as is produced by respiration during the 

 same time, so that when both processes are going on there is always 

 a decrease in the carbon dioxide and an increase in the oxygen 

 of the atmosphere, and only at night or in the dark does the 

 process of respiration become apparent. However, in parts 

 of plants which are not green, such as the roots, flowers 

 and germinating seeds, respiration is readily detectable at all 

 times. 



The carbon compounds which disappear while respiration is 

 going on, are carbohydrates, such as starch and the various 

 sugars and fats. The oxidation of these substances does not 

 take place at ordinary temperature outside the plant, and the 



