REPORT ON THE PHILOSOPHY OF BOTANY. 55 



producing a green deposit in the parenchyma, is in proportion 

 to their illuminating property ; that no decomposed rays effect 

 this so rapidly as white light; and that the yellow ray possesses 

 the greening power in the highest degree, the orange in a very 

 slight degree, and violet, red and purple not at all. 



Colours. — Nothing can be named in the whole range of bo- 

 tany upon which information is so much wanted as the cause of 

 the various colours of plants. It was, indeed, long since sus- 

 pected by Lamarck that the autumnal colouring of leaves and 

 fruits was a morbid condition of those parts ; and it has subse- 

 quently been ascertained that all colours are owing to the pre- 

 sence of a substance, called chromiile by De CandoUe, which 

 fills the parenchyma, assuming different tints. Green has also 

 been clearly made out to be connected with exposure to light, 

 and has been considered to be in all probability owing to the 

 deposition of the carbon left upon the decomposition of car- 

 bonic acid. Some botanists have also observed the connexion 

 of red colour with acidity ; but still we had scarcely any positive 

 knowledge of the cause of the production of any colour except 

 green, till M. Macaire of Geneva* remarked, that just before 

 leaves begin to change colour in the autumn, they cease parting 

 with oxygen in the day, although they go on absorbing it at 

 night ; whence he concluded that their chromule is oxygenated, 

 by which a yellow colour is first caused, and then a red, — for he 

 found that in all cases a change to red is preceded by a change 

 to yellow. He also ascertained that the chromule of the red 

 bracteae and calyx of Salvia splendens is chemically the same 

 as that of autumnal leaves. Coupling this with the fact that 

 petals do not part with oxygen, it would seem as if their colour, 

 if yellow or red, may also be owing to a kind of oxygenation. 

 But according to M, Theodore de Saussure f , coloured fruits 

 part with their oxygen ; so that, if this be true, red and yellow 

 cannot always be ascribed to such a cause. M. De CandoUe J 

 has some excellent observations upon this subject in his recent 

 admirable digest of the laws of vegetable physiology ; in which 

 he concludes, from the inquiries hitherto instituted, that all co- 

 lours depend upon the degree of oxygenation. When oxygen 

 is in excess, the colour seems to tend to yellow or red ; and 

 when it is deficient, or when the chromule is more carbonized, 

 which is the same thing, it has a tendency to blue. Local ad- 

 ditions of alkaline matters are also called in aid of an explanation 

 of the various shades of colour that flowers and fruits present. 



* Mcmoirrs de la Socictc Physique de Geneve, vol. iv. p. 50. 

 t Ibid. vol. i. p. 284. % Phjsiologie Vegetale, p. 906. 



