174 PHYSIOLOGICAL BOTANY. 



and at night especially oxygen is consumed and car- 

 bon dioxide is set free. This fact has led to the in- 

 ference that 



Potted plants in a living room render the air unfit 

 to breathe ; but carefully conducted experiments have 

 shown that one hundred ordinary stove plants would 

 not injure the air of a moderate sized sitting or living 

 room to an extent that could be in any way injurious. 



429. Metabolism is the name applied to the process 

 which goes on in the structure of living plants that 

 alters one kind of material of plant growth into an- 

 other ; an example of which is the change of starch 

 into cellulose. 



430. Assimilation is the process of taking into the 

 plant's structure surrounding substances and convert- 

 ing them into materials for plant growth, and consists 

 mainly in changing inorganic substances into vegetable 

 structure. The bulk of all woody plants is largely com- 

 posed of carbon, hence assimilation in such plants con- 

 sists mainly in disintegrating carbon dioxide, and ap- 

 propriating the carbon. Assimilation is carried on in 

 the cells of the green tissue and in sunlight. 



Some of the substances suspended in the watery 

 fluids of plants and the constituents of water itself are 

 used directly by the protoplasm in the preparation of 

 food ; carbon dioxide, however, must first be decom- 

 posed, in which process its oxygen is set free, and the 

 carbon enters into the ligneous structure, or both oxy- 

 gen and carbon enter into new combinations which the 

 protoplasm can use. For example, water and carbon 

 dioxide contain all the materials found in starch. 

 These compounds having been separated into their 

 constituents, the elements reunite in quantities that 



