THE LIBERATION OF ENERGY 55 



sunlight, carbon dioxide united with water, i.e., 

 carbonic acid, is, as we have already seen, decomposed, 

 and photosynthesis of primary organic compounds 

 takes place. It will be at once manifest that this 

 nutritive process must mask the respiratory process, 

 if the amount of carbon dioxide required for photo- 

 synthesis be greater than that formed in respiration, 

 and that the formation and excretion of carbon 

 dioxide will not be apparent ; the carbon dioxide 

 will be decomposed and rebuilt by the green cells as 

 soon as it appears in their vicinity. For that reason 

 the green plant appears not to be respiring during 

 the day ; on the contrary, it appears to be giving 

 off oxygen by day, and carbon dioxide by night. 

 But this is easily explicable if we remember that 

 during the night no photosynthesis is going on 

 although respiration is, whilst during the day, 

 although both photosynthesis and respiration are 

 taking place, the carbon dioxide required for photo- 

 synthetic purposes so much exceeds in quantity the 

 carbon dioxide produced by the tissues that none of 

 the latter is able to escape, whilst, from it, as well 

 as from the surplus carbon dioxide taken in, oxygen 

 is released and exhaled as a by-product in photo- 

 synthesis. Hence the statement often made that 

 one of the essential differences between a plant and 

 an animal is that " whilst the animal takes in oxygen 

 and gives off carbon dioxide, the plant takes in 

 carbon-dioxide and gives off oxygen." Both state- 

 ments, as a matter of fact, are perfectly correct, 

 but the actual comparison is entirely misleading, 

 since the taking in of carbon dioxide and giving off 

 of oxygen is a nutritive or constructive process, 

 whereas the converse process is respiratory or de- 

 structive. That respiration takes place in green plants 



