ATMOSPHEllIC AlK As TlUi FoOD OF PLANTS. 2f» 



have no power to increase in mass at the expense of the 

 atmosphere and soil ; tliey liave no provision for the ab- 

 sorption of the nutritive elements that surround them ex- 

 ternally, but grow at the expense of other parts of the plant 

 (or seed) to which they belong. The function of free 

 gaseous oxygen in vegetable nutrition, so far as c;m be 

 judged from our existing knowledge, consists in effecting 

 or aiding to effect the conversion of the materials which 

 the leaves organize or which the roots absorb, into the 

 proper tissues of the growing parts. Free oxygen is thus 

 probably an agent of assimilation. Certain it is that the 

 free oxygen which is absorbed by the j^lant, or, at least, a 

 corresponding quantity, is evolved again, either in the un- 

 combined state or in union witli carbon as carbonic acid. 

 Exhalation of Oxygen from Foliai^e.— The relation of 

 the leai^es and green parts of plants to oxygen gas has 

 thus far been purposely left unnoticed. These organs like- 

 wise absorb oxygen, and require its presence in the atmos- 

 phere, or, if aquatic, in the water which surrounds them ; 

 but they also, during their exposure to lights exhale oxygen. 

 This interesting fact is illustrated 

 by a simple experiment. Fill a 

 glass funnel with any kind of fresh 

 leaves, and place it, inverted, in a 

 wide glass containing Mater, fig. 

 3, so that it shall be completely 

 immersed, and displace all air from 

 its interior by agitation. Close the 

 neck of the funnel air-tight by 

 a cork, and pour off a portion 

 of the water from the outer vessel. Expose now the 

 leaves to strong sunlight. Observe that very soon minute 

 bubbles of air will gather on the leaves. These will 

 gradually increase in size and detach themselves, and 

 after an hour or two, enough gas will accmnulate in 

 the neck of the funnel to enable tlie experimenter to 

 2 



