42 PLANT LIFE. 



supplies of reserve-material ready for use when growth 

 commences. 



During the chemical changes that take place in reserve- 

 material previous to its being used up in the formation of 

 new parts, certain substances are formed that are of no use 

 to the plant in the formation of cells ; these are called by- 

 products of metabolism. Amongst others may be mentioned 

 volatile oils, tannin, oxalate of lime, etc. Such substances 

 are often used for attractive and protective purposes. A 

 prevalent idea in connection with the processes of plant life 

 is that plants purify the atmosphere, that is, from the animal 

 point of view, by removing the carbonic dioxide and supply- 

 ing oxygen, which is perfectly true, the mistake arising in 

 usually connecting this function with respiration, which is 

 not correct. The interchange between the gases of the 

 atmosphere and those contained within the plant, also the 

 chemical changes produced in them, are due respectively to 

 two distinct functions, respiration and assimilation ; and if 

 this fact is thoroughly grasped the apparent contradictory 

 statement, that in the performance of the first-named func- 

 tion oxygen is removed from the atmosphere and carbonic 

 dioxide added, while in the second carbonic dioxide is 

 removed and oxygen added, becomes intelligible. 



Respiration or breathing in plants is a constant process, 

 and quite independent of light, and as in the animal king- 

 dom is for the purpose of purifying the organism. For this 

 purpose oxygen is inhaled from the atmosphere and again 

 exhaled in chemical combination with the refuse carbon 

 under the form of carbonic dioxide, often spoken of as 

 carbonic acid (CO^). Again, as in the animal kingdom, a 

 certain amount of heat is set free during respiration, which 

 is a process of oxidation ; but as respiration is usually con;- 



