Philosophical Character of Decandolle. , 2o7 



common interest, and his masterly chapter on this subject 

 might, we should think, if properly studied, have removed, 

 from the minds of certain German physiologists, the doubts 

 they appear at a much more recent period to have entertained, 

 with respect to the source of the carbon existing in plants, 

 and also perhaps have induced an illustrious living chemist of 

 that country,* to limit to the naturalists of his own nation the 

 strictures, which he seems to consider applicable to the general 

 body of those in Europe, for being, as he conceives, so far in ar- 

 rear of the actual state of chemical knowledge in this respect. 



Nevertheless he still leaves undecided, what the descrip- 

 tion of rays may be which acts most favourably in promoting 

 vegetation, whether, for instance, the chemical or the lumi- 

 nous portion of the spectrum is most efficient ; neither has he 

 adduced from De Saussure a sufficient amount of evidence to 

 overpower the weight due to the experiments of Mr Ellis, 

 which tended to throw doubts upon the extent of the purify- 

 ing influence attributable to plants. 



I am, therefore, induced to flatter myself, that theresearchest 

 in which I have myself been subsequently engaged, with a 

 view to the more satisfactory elucidation of these two points, 

 would not have been regarded by this great botanist as alto- 

 gether superfluous. 



Decandolle has also presented us with a very ingenious 

 theory with respect to the autumnal coloration of leaves and 

 fruits, which he regards as due to an action exerted upon their 

 colouring matter, by an acid generated within their tissue, 

 when the latter has begun to lose, with its vitality, the power 

 of separating carbon from oxygen. | 



He has also laid down a curious law with respect to the 



* See Liebig's remarks, in his Eeport " On Chemistry in its application 

 to Agriculture and Physiology," 2d English edition, p. 30. 



t See my Paper in the Piiilosophical Transactions for 1836, " On the action 

 of Light upon Plants, and of Plants upon the Atmosphere." 



\ The new photographic researches of Sir John Herschel on vegetable 

 colours promise to lead to a better understanding of this subject. He finds 

 leaves to contain two colours, the green destructible by light, the red not. 

 Hence, when the activity of the vegetation no longer recruits the green prin- 

 ciple, in the same ratio in which it is destroyed by the solar influence, the 

 red predominates, and the sere or withered appearance of the leaf super- 

 venes. — See Philosophical Magazine for February 1843. 



