1887-88.] Colouring Matters of Leaves and Flowers. 279 



from chlorophyll or etiolin. Tn many cases red, especially 

 where tannin is present, is known to result from the decom- 

 position of glucosides, for example, in certain roots. 



Blvx is very rarely fixed. It is probably a derivative of 

 red in all cases. 



Violet, like orange and scarlet, is produced by the mix- 

 ing of certain of the colours already mentioned in the same 

 tissue, for instance, where cells containing fixed yellow have 

 fluid red in the superposed cells. 



Black is a result of tlie concentration of violet pig- 

 ment. 



Brown is due to amorphous decomposition-products, or to 

 the presence of red and yellow pigments in the cells. 



Variegated colours are often due, according to Engelmann, 

 to localised colours in the walls of the cells. 



A physical property of chlorophyll as of allied colours 

 is its fluorescence. This property is not shared by etiolin, 

 from which, as we have seen, all the chlorophyll-colours 

 are produced. It is a property of many vegetable colours, 

 and may be artificially produced by the addition of alcohol 

 to the colourless fluid of the pellucid spots of Hypericum, 

 which become brilliantly red as well as fluorescent. 



At the present time a large amount of information regard- 

 ing colours, as observed through the spectroscope, has been 

 collected. Etiolin, chlorophyll, and xanthophyll have each 

 a characteristic spectrum. The position of the absorption- 

 bands in chlorophyll is materially changed when there is the 

 least trace of acid present ; similarly they are distinctively 

 changed with an alkali. 



More or less suggestive facts, such as the following, have 

 been obtained — that the spectrum of fresh leaves submitted 

 to sunlight changes in appearance, and ends by giving the 

 spectrum of chlorophyll that has been acted upon by acids ; 

 that the spectrum of the yellow matter of dead leaves and 

 of certain flowers is the same as that of xanthophyll ; that 

 the processes of assimilation and transpiration are variously 

 modified according as they are effected in rays from one or 

 another part of the spectrum. 



It must be borne in mind, however, as was pointed 

 out by M. Chaubard, in 1873, that the spectrum of chlorophyll 

 may vary in the same plant, according to age, climate, tem- 



