49 



solution of this substance in alcohol is so precipitated by water 

 that a pale yellow milk is formed. Xanthophylle is dissolved 

 by aether and becomes brown in concentrated sulphuric acid. 

 Respecting the formation of the xanthophylle from chloro- 

 phylle M. Berzelius observes, " We have every reason to sus- 

 pect, that with the disappearance of the green colour and its 

 change into yellow, the xanthophylle is produced from the 

 chlorophylle by a change of the organization of the leaf ef- 

 fected by cold, and the alteration in the organic process thus 

 caused." It was not possible to produce this change artificially. 



The brown colour of the foliage has nothing, says M. Ber- 

 zelius, in common with the yellow ; it is produced by an ex- 

 tract at first colourless, which becomes brown by the action 

 of oxygen ; yet it may also be observed in this latter case, that 

 all the cellular membrane of the leaf has acquired a brown 

 colour which cannot be extracted even by digestion in weak 

 alkaline solutions. The various proportions in which this brown 

 extract occurs with the xanthophylle give the autumnal co- 

 lours of the leaves a number of tints. 



Notwithstanding all the beautiful researches which have 

 been made of late years on the formation of colours in plants, 

 both in anatomical as well as chemical respects, we are yet very 

 far from the solution of this enigma, as we shall perceive im- 

 mediately from the following observations. 



M. J. Decaisne* has published a very elaborate examina- 

 tion of Rubia tinctorum, and allied species, which are cul- 

 tivated for the purpose of obtaining the madder or red colour. 

 I shall have occasion to notice this work frequently, but shall 

 here only mention those observations which relate to the for- 

 mation of the well-known colouring matter of these plants. 



M. Decaisne is convinced from numerous observations that 

 the colouring substance in Rubia tinctorum, and in general in 

 the Rubiacece, does not occur in peculiar vessels or secretory 

 organs, but in the interior of the elementary organs. It is 

 usual to employ only the root in the preparation of the colour- 

 ing substance, in which it occurs in great abundance ; if how- 



* Recherches anatomiques et physiologiques sur la Garance, sur le de*- 

 veloppement de la matiere colorante dans cette plante, sur sa culture et sa 

 preparation, suivies de 1'examen botanique du genre Rubia et de ses especes. 

 Bruxelles, 1837, 4to. avec 10 pi. 



E 



