ANAT()?;iV OF P1.A>.TS. 201 



deprived of its carbonic acid, is exposed to the light of the 

 sun. Universally this effect ceases when the leaves begin to 

 fade, to become discoloured, and to fall. It is most power- 

 ful in leaves which fall periodically, because their irritabi- 

 lity is considerably greater than that of evergreen and fleshy 

 leaves. This effect is, in the last place, most actively produ- 

 ced, when the electrical excitation of the atmosphere is great- 

 est, on which account the greatest quantity of oxygen is ex- 

 haled from leaves during Spring, after a storm, and in the 

 morning. 



317. 



But the exhalation of oxygen gas is closely connected 

 with a remarkable property of leaves, namely, their green co- 

 lour. As this colour in the rainbow stands exactly in the 

 middle, between the two outermost tints, the red and the vio- 

 let, — as it is bounded on the one side by the yellow, and on 

 the other b}^ the blue, — as all experiments further shew that 

 the red and yellow tints are more of an oxygenous, and tlie 

 blue and violet more of a hydrogenous nature, — it is extreme- 

 ly probable, that the green colour is the effect of a neutrali- 

 zation between the two extreme colours, or that it arises, when 

 the light has attracted exactly as much oxygen as was requi- 

 red by the hydrogen and carbon which remained. And this 

 theory seems to be conffrmed by the following observations. 

 All plants, so long as they are withdrawn from the light of 

 the sun, are of a pale yellow colour, and regain this same hue 

 when, as in the instance of the Endive and Cardoon, they 

 have been covered with earth and blanched. In this condi- 

 tion, they are rich in oxydized juice, as theii' sweet taste, and 

 the tenderness of their parts, shew. Besides, these blanched 

 plants give out nothing but carbonic acid water, saccharine 

 matter, and mucilage. As soon, however, as the light of the 

 sun has called forth the proper activity of the plant, it emj)- 

 ties itself of its superfluous oxygen, and forms those partly 

 resinous, partly oily substances^ which we find connected 

 with the green colouring matter. The green colouring mat- 

 ter evinces its resinous nature by this circumstance, that it 



