76 BIOLOGY OF PLANTS. 



is known, " it is a matter of the greatest ease and certainty to 

 calculate which of them, during life, should absorb most 

 oxygen by chemical action when the influence of light is 

 withdrawn." The oxygen in this casecombines with the vol- 

 atile oils, and changes them into resins, in some cases, while 

 in others, it unites with the constituents of nut-galls, and 

 forms acids, or with substances containing nitrogen. 



The carbonic acid in this case is derived from the sap ; it 

 enters the roots with the water, and when it arrives at the 

 leaves it is not decomposed, but is transpired along with the 

 water ; this, of course, is purely a mechanical process, and 

 the quantity of acid will depend on the quantity of water. 

 The absorption of the oxygen and the emission of carbon- 

 ic acid have no connection therefore with each other, or 

 with the process of assimilation. A cotton wick, in a 

 lamp filled with water, saturated with carbonic acid, acts pre- 

 cisely like a plant in the night ; water and carbonic acid are 

 sucked up, and evaporated from the wick. 



The quantity of oxygen absorbed by plants depends upon 

 their vigor, degree of heat, and the nature of their leaves. 



1. The more vigorous the plant is, the greater the quantity 

 of oxygen which it is capable of absorbing during any given 

 period. This we should expect, because all the vital forces 

 are more active, and hence the growth must be more rapid, 

 and require a larger supply of the appropriate nutriment. 



2. The same species of plants will absorb more oxygen at 

 a temperature of 88° F. than at 55° or 56° F.* This quan- 

 tity will therefore depend upon the season of the year and 

 upon the climate. 



3. The leaves of different plants do not consume the same 

 quantity of oxygen gas, at the same temperature, and seasons 

 of the year. The quantity varies from a little more than one 

 half the bulk of the leaves, to eight times their volume. 



The fleshy-leaved plants absorb the least oxygen, and re- 



* Chaptal. 



