72 STUDY OF COMMOK PLANTS. 



Still another function which the leaf shares vdth other 



living parts of the plant, and wh^ch is characteristic of all 



living cells whether plant or animal, is that of 



spira ion. jegpjpation. As we have seen, one of the prod- 

 ucts of respiration, carbon dioxide, is easily demonstrated 

 by testing with limewater the air within a bottle contain- 

 ing a quantity of'green leaves. The abundant precipitate 

 of carbonate of lime shows that the leaves are giving off 

 carbon dioxide in considerable quantity, and as this is true 

 whether the experiment is performed in the daytime or at 

 night, we infer that respiration is going on continually. 

 It should be said, however, that, contrary to a widely 

 spread popular belief, the quantity of carbon dioxide 

 exhaled by plants is so small in comparison with what is 

 given off in animal respiration that it may be disregarded 

 in connection with the question of keeping house plants. 

 They are a decided advantage in the home from a sanitary, 

 as well as aesthetic, point of view. 



The chief functions of the leaf, then, are 



1. Assimilation, or the production of organized material. 



2. Transpiration, or the evaporation of water that has 

 served as a vehicle for the transportation of crude sub- 

 stances. 



3. Respiration, a process common to all living things. 



The first of these takes place in sunlight, or its equiva- 

 lent ; the second is most active in the daytime, but is not 

 limited to it ; and the last continues both day and night, 

 as long as the leaf is alive. 



We have learned in our study of the barberry and a 

 number of other familiar plants, that leaves are subject to 

 various modifications corresponding to other than their 

 ordinary functions. These modifications are not infre- 



