S72 M. Macaire Prinsep on the Cohur'ing of the- 



ly sheltering from the action of hght, either the whole branches, 

 or parts of leaves. 1 always found that, in this deprivation, all 

 change of colour was prevented. If the entire leaf was placed 

 in the dark, it fell off green ; if only a part, the rest of the pa- 

 renchyma changed colour, and the covered portion retained its 

 original colour. I satisfied myself further, that light was neces- 

 sary in all the phases of the phenomenon, and if I placed in 

 the dark leaves or portions of leaves which were yellow before 

 reddening, as the Rhus coriaria, the leaf fell off yellow, or the 

 covered part retained that colour, while the rest became red ; 

 which demonstrates the necessity of the action of light in all the 

 stages of colouring. 



Action of the Atmospliere. — Every body knows, and it is es- 

 pecially to the beautiful experiments of M. de Saussure, that 

 the demonstration of this important fact is owing, that the green 

 parts of plants absorb during the night a variable quantity of 

 oxygen, according to the different species, and that they expire 

 a certain proportion of that gas when they are exposed to the 

 sun in spring water. Being curious to know the modifications 

 which the autumnal colouring of the leaves might induce in this 

 phenomenon, I made several series of experiments, following the 

 indications of M. de Saussure in the most scrupulous manner 

 possible. I satisfied myself in the first place, that the already 

 coloured leaves do not disengage oxygen gas by exposure to the 

 sun^s light, and 1 afterwards learned that this fact had been as-^ 

 certained by M. Senebier. Pushing my inquiries farther, I 

 found, that, when the leaves were either coloured in part, or at 

 the point of changing colour, even although they yet appeared 

 green to the eye, they, from tliat moment, ceased to give out oxy- 

 gen in the sun. I also found, by a great number of trials, the 

 particulars of which I intend to make known, that the leaves, 

 on arriving at the very point where the tendency to the autum- 

 nal colouring commences, continued to inspire oxygen gas dur- 

 ing the night, and in a quantity always decreasing in proportion 

 as the colouring advanced ; which allowed me to conclude tliat 

 it was to the fixation of the oxygen in the colouring matter of 

 the leaf that the change of tint was owing. 



Of the colouring principle of the Leaves. — Some years ago, 

 MM. Pelletier and Cavantou discovered that the green substance 

 of the leaves possesses peculiar properties, and ranked it, under 



