430 Dr, Gardner's Researches on the Functions of Plants. 



Scheele, Cruickshank and others. Gough's interesting ob- 

 servation, that plants grown in darkness do not become green 

 in light unless oxygen be present, is accounted for by the 

 fact that chlorophylle is an oxidized product. 



19. The movements occurring during light owe their con- 

 tinuance to its action. The gases absorbed are destined to 

 equilibrate the internal atmosphere, but cannot effect this ob- 

 ject so long as carbon and oxygen are fixed by the plant. 

 Hence the current becomes continuous during daylight, the 

 oxygen and carbon being removed faster than they penetrate. 



20. But during darkness the stream is arrested, carbonic 

 acid is no longer drawn from the air, but often evolved, as ob- 

 served by Ingenhousz and Saussure. Oxygen is for a time 

 required to satisfy chemical affinities, and afterwards the in- 

 ternal gas resembles atmospheric air ; 20'6 per cent, oxygen 

 is found, and amounts of nitrogen and carbonic acid differing 

 with the nature of the soil-fluid, the latter gas sometimes rising 

 to 3 per cent., when it is evolved ; and at others not attaining 

 a proportion much higher than that of the air, when it is not 

 thrown off during night, as shown by Mr. Pepys. 



21. It is the decomposition of the carbonic acid within the 

 plant by the early sun-beams which creates the absorption 

 from without by deranging the internal atmosphere. Car- 

 bonic acid being decomposed, a temporary excess of oxygen 

 is produced, which causes a portion to pass outwards; but in 

 preparing the plant for examination, any excess of this body 

 seems to have been removed, so that it is probable that in the 

 living organism oxygen gas is a much more important element 

 than is usually admitted by physiologists. 



V. The Action of Plants on Artificial Atmospheres. 



22. To show beyond a doubt that the penetration of gases 

 into plants is a physical and not vital process, we adduce the 

 effects of artificial atmospheres. In these experiments the 

 gases given out and absorbed are not of a definite mixture, 

 but depend altogether on diffusion. 



23. M. Marcet placed the same fungi in atmospheres of 

 common air, oxygen, and nitrogen; and after eight to ten 

 hours they changed 100 measures of — 



into Oxygen . . . 

 Nitrogen . . . 

 Carbonic acid . 



100-0 100*0 100-0 



without alteration of volume (Ann. de Chimie, tyc, t. lviii. 



