iS73-J Comparative Vegetable Chroma tology. 465 



destroyed, so as to make way for the younger and more 

 active. It will also be requisite to still further study the 

 variations in the spectra of the different colouring matters 

 which depend upon the conditions in which they occur, 

 since in some cases it is thus possible to ascertain whether 

 they naturally exist in a free state or are combined with 

 oily or waxy substances. This makes such a remarkable 

 difference in the spectra of some yellow flowers, that for 

 a long time I thought that the colouring matters were 

 entirely different, but I have now found that when oil pene- 

 trates to the endochrome, so as to combine with the colour- 

 ing matter, the spectrum is changed to exactly the same as 

 that met with in other cases which are not thus changed 

 by the addition of oil, as though sufficient had been 

 naturally present. In these experiments, the petals must 

 be well crushed, so as to burst open the cells, and then 

 dried, or else the oil will not penetrate to the endochrome. 

 By carefully examining the position of the absorption- 

 bands, we may not only determine such facts as these in 

 the case of a colouring matter insoluble in water, but when 

 they are soluble we may sometimes prove that they are not 

 dissolved in the aqueous juices in the living plant, but 

 do become dissolved when decomposition takes place, as 

 though perhaps originally enclosed in minute cells, and 

 set free when the cell-walls decay. Independent of these 

 various general questions, the study of all the leading 

 classes of plants, and of a number of the lower classes of 

 animals, is necessarily a very extensive subject that can 

 only be worked out by degrees, on account of the difficulty 

 of procuring the requisite materials, in a fresh state, at the 

 proper season of the year ; and it is made still more 

 extensive by the necessity of examining the same plants at 

 different periods of their growth, and when grown under 

 different natural and artificial conditions. On the whole, 

 then, it will be seen that comparative vegetable chro- 

 matology, in its full extent, including everything requisite 

 for its successful investigation, is a very wide and almost 

 new branch of science ; and, though I have accumulated a 

 large amount of facts, I cannot but feel that it is only. in its 

 infancy. Still, however, the brief account of it which I 

 have now given will, I trust, suffice to show that even 

 already it has thrown a new light on a number of facts, and 

 that a further and more complete study will probably 

 enable us to examine some of the most important questions 

 connected with vegetable physiology and the evolution of 

 plants, from a new and independent point of view. 



