238 LECTURE XII. 



Fungi on dilute solutions of aromatic substances such as 

 carbolic, salicylic, and benzoic acids (p. 125). Taking all 

 this into consideration, we may venture upon the general 

 statement that the higher plants, at least, cannot avail them- 

 selves of carbon when combined in an aromatic molecule for 

 the purposes of their constructive metabolism. 



Although the aromatic substances are probably to be 

 regarded simply as waste-products, yet some of them are 

 indirectly of use to the plant. We have seen that the 

 odours of plants are due to the presence of volatile ethereal 

 oils, and it has been ascertained that the odours of flowers 

 serve to attract insects and thus contribute to ensure fertili- 

 sation. 



We have yet the -group of colouring-matters to consider. 

 These may be conveniently classed as follows : 



1. Phlobaphenes, or colouring-matters of the bark. 



2. Colouring-matters of woods, etc. 



3. Colouring-matters of leaves, flowers, and of Thallophytes. 



The phlobaphenes are brown amorphous colouring-matters which are 

 present in the walls of the bark-cells of trees and shrubs. They closely 

 resemble the brown products of the oxidation of tannin and other 

 glucosides, and it seems probable that they are in fact formed in the 

 plant in this way. When they are fused with caustic potash they yield 

 protocatechuic acid as one of the products, and this acid is also formed 

 when the brown products of the oxidation of tannin are treated in the 

 same way (Hlasiwetz). 



The colouring-matters of woods occur, like the phlobaphenes, in the 

 cell-walls, and, although nothing is definitely known as to their origin, 

 it is probable that they are formed in much the same way. There is, 

 however, reason for believing that they are not formed in the cell-wall, 

 but in the cavity of the cell, and that as the cells lose their contents and 

 become dry the colouring-matters are taken up by the cell-wall. Such 

 are Brasilin, C 6 H 14 O S +H 2 O, obtained from Brazil-wood (Ccesalpinia brasi- 

 liensis), and Haematoxylin, Ci 6 H 14 O 6 + 3H 2 O, obtained from the wood of 

 Hamatoxylon campechianum. 



In some plants glucosides are present which, after extraction, undergo 

 decomposition, giving rise to colouring matters. Thus the root of Rubia 

 Tinctorum (Madder) contains a substance termed ruberythric acid, 

 C^H^Oi.!, which decomposes, under the action of a ferment contained 

 in the root, into a colouring matter, alizarin, and glucose, according to the 

 equation, 



H + 2H 2 = Q 4 H 8 4 + 2 (C 6 H 12 6 ). 



