208 STRUCTURAL BOTANY 



plant (such as the Elodea, mentioned on p. 41) are put 

 into a glass of ordinary water, which always contains 

 some carbon dioxide in solution. The glass with the 

 Water-weed in it is allowed to stand in a sunny place, 

 and we soon see that bubbles are given off from the 

 plant in a constant stream. If we catch these 

 bubbles, as they rise to the surface, in an inverted 

 test-tube filled with water, we find that they consist 

 chiefly of oxygen, as can be easily shown by plunging 

 a glowing match into the gas thus collected; the 

 match will at once catch light again. If, however, the 

 plant be placed in water which has been previously 

 deprived of its carbon dioxide by boiling, no oxygen 

 will be given off. 



If a plant, or simply some green leaves, be placed 

 in a bell-glass, dipping into mercury below, and con- 

 taining a measured quantity of carbon dioxide, and 

 left for some time exposed to light, the volume of the 

 gas in the jar remains unaltered. If the gas be 

 analysed after some hours, the carbon dioxide will be 

 found to have diminished, and the oxygen to have 

 increased, and in fact the volume of carbon dioxide 

 lost will be exactly equal to the volume of oxygen 

 gained. In other words, the plant has given out 

 exactly as much oxygen as it has absorbed carbon 

 dioxide. This seems at . first to prove that all the 

 oxygen of the carbon dioxide is given off, and only 

 the carbon retained by the plant. We shall see 

 presently, however, that another explanation is more 

 probable. 



The decomposition of so stable a compound as 



