442 CHEMICAL DISCOVERY AND INVENTION 



plant is not a mixture of two or more closely similar compounds. 

 Supposing this to be the case the constitution of the several 

 modifications is of the same type, and the differences may arise 

 from mere differences of arrangement of the constituent atoms 

 in space. Molecules so complicated as those of chlorophyll are 

 open to many possibilities of this kind. 



In view of the general prevalence of chlorophyll and the 

 change of the green bud into the coloured flower as well as the 

 frequent tendency to reversion of coloured parts to the green 

 state, it might be supposed that the various bright hues of flowers 

 were produced by some kind of chemical transformation of 

 chlorophyll. This idea was certainly accepted at one time, but 

 it appears to be without foundation. 



Many vegetable colouring matters were extensively used as 

 dyes before the discovery of the coal-tar colours, and a few still 

 retain their position. It is only necessary to remind the reader 

 that, notwithstanding the advent of the very numerous synthetic 

 dyes, natural indigo, logwood, safflower, and madder colours are 

 still used to some extent in the dye house, and that the colouring 

 matters of the damask rose, the red poppy, turmeric root, litmus, 

 and red cabbage are employed in other ways. The diversity of 

 natural colouring matters is a very interesting fact. In many 

 cases it is probably true that the accumulation of colouring 

 matter, especially in the coverings of the flower, is connected 

 with a definite advantage to the plant. The bright coloured 

 corolla, often associated with the secretions of essential oil 

 which fills the surrounding air with perfume, brings the visits of 

 insects which in many cases play an essential part in the process 

 of fertilisation. On the other hand, there are many coloured 

 substances secreted in the inner parts, in woods, like the barberry 

 and logwood, in roots, like the turmeric and rhubarb, in bark, as 

 in the quercitron (Quercus tinctoria), in berries, such as those of 

 buckthorn and various other species of Rhamnus. In such 

 cases the advantage, if any, must be of a different kind. It seems 

 probable that, in many cases at any rate, the colouring matters 

 thus deep-seated are, like many of the other chemical com- 

 pounds found in the tissues of plants, merely waste pro- 

 ducts concomitant with those which must be regarded as 

 essential. 



A plant requires, for example, to manufacture the chemical 

 compound cellulose of which the membranous walls of its cells 



