RESPIRATION 271 



which heat is generated. The fact that green plants 

 absorb carbon dioxide in sunlight, and in the process of 

 carbon assimilation liberate oxygen as a waste product, 

 is apt to obscure the equally important fact that they 

 also, both by day and by night, take in oxygen and throw 

 off carbon dioxide. The oxygen inhaled oxidizes tissues 

 and liberates energy essential to the life of the plant, 

 while the exhaled carbon dioxide is a by-product of 

 tissue oxidization. We used to be told that to have 

 plants in a room by night was dangerous to health, but 

 that they were a source of health in daytime. The 

 fiction was evidently based upon the idea that, because 

 carbon dioxide is absorbed only in sunlight, it could be 

 only in daytime that they could purify the atmo- 

 sphere. True is it that carbon assimilation ceases at 

 night, but it is equally true that oxygen is inhaled and 

 carbon dioxide exhaled day and night throughout the 

 life of a plant. It is not advisable in a book of this 

 character to discuss the technicalities of plant respira- 

 tion; they are somewhat abstruse, and the reader who 

 is prepared to study them will refer to technical text- 

 books for such information as is available. It will 

 suffice the ordinary reader to know that plants not only 

 drink and eat, they also breathe. 



So far in our inquiry into the nutrition of plants we 

 have learned what elements are essential for healthy 

 plant life, and the sources from whence these elements 

 are obtained. We know the elements are present in 

 chemical compounds, and are not absorbed in an uncom- 

 bined state, and it has been plainly stated that the food 

 absorbed in an inorganic form is transformed by the 

 alchemy of protoplasts into organic compounds. It is 



