Dr. Gardner's Researches on the Functions of Plants. 429 



15. The gas of the pump-water was N 48, O 22, C0 2 30 

 per cent., therefore the roots absorbed carbonic acid and oxy- 

 gen in the same way as a porous system containing the nor- 

 mal plant atmosphere, N 86*75, O 13*25 per cent.; this con- 

 tinued during daylight, or during the activity of the vegetable, 

 but in darkness all the gas of the water is taken up without 

 any selection. 



IV. The Absorption of Gases by Plants is a consequence of 

 their Porosity. 



16. We are now in possession of sufficient data to state the 

 case. A porous system lies between two media and contains 

 a certain mixture of gases ; the gases of the three systems are 



The air. The plant. gas. The water-gas. 



Carbonic acid . 0*05 0-00 30'00 



Oxygen . . . 20*80 13*25 22*00 



Nitrogen . . . 79*15 86*75 4800 



100*00 100*00 100*00 



If the plant-gas were confined within an extremely delicate 

 caoutchouc bag and surrounded by either atmosphere, it would 

 soon be disturbed by penetration, nitrogen would be evolved, 

 and carbonic acid and oxygen absorbed. The rapidity of the 

 interchange would depend on the gas and the nature of the 

 membrane. 



17. That the action of roots on the water-gas is coincident 

 with this theoretical view, I have attempted to show in the 

 last division of the subject. The experiments detailed in 

 article 5 were also made with this object. It must be re- 

 membered that the operation of the plant atmosphere upon 

 the gases of the soil is not direct, as in the leaves, but through 

 the intervention of the sap, which contains a mixture of gases 

 dependent upon those of the interior of the plant, but has a 

 greater capacity for carbonic acid and oxygen than for nitro- 

 gen. 



18. In the case of leaves, the physical theory of porosity is 

 more strikingly made out, because the internal gas is here in 

 contact with atmospheric air, the epidermis only lying be- 

 tween. The movements witnessed in the experiments of art. 

 5, represent those of a vigorous plant in sun-light; carbonic 

 acid and oxygen are absorbed and nitrogen evolved. It is not 

 uniformly admitted that oxygen is absorbed and nitrogen 

 evolved in plants, but an investigation of this point leaves us 

 under the conviction that the negative is untenable. Saussure, 

 DeCandolle, Palmer, Daubeny, Draper and others have wit- 

 nessed the evolution of nitrogen. The continued absorption 

 of oxygen is confirmed by Saussure, Davy, Gough, Achard, 



