326 COLOURS OF PLANTS. 



in autumn, while such as produce white grapes have yellow leaves 

 These yellow and red colours by some are said to depend on changes, 

 in the state of oxidation of the chlorophylle, and have been traced by 

 others to the production of peculiar waxy substances, one red, called 

 crythrophylle, the other yellow, xanthophylle. Marquart believes that 

 the action of water on chlorophylle, in different proportions, gives rise 

 to yellow and blue matters. Ellis supposed the change of hues to be 

 due to the prevalence of acid and alkaline matters. 



676. Dr. Hope endeavoured to show that there is in plants a 

 colourable principle, chromogen (^uf**, colour, and yewu.u t I generate), 

 consisting of two separate principles, one of which forms a red com- 

 pound with acids, while the other forms a yellow with alkalies, and he 

 attributes the green colour produced by the latter to the mixture of 

 the yellow matter with the blue infusion. The two principles, 

 according to him, may exist together, or separate, in different parts of 

 the same plant. In some very fleshy leaves, as Agave, the central 

 cells are pale, while those of the cuticle are coloured and much 

 thickened. Although leaves are usually of a green colour, still they 

 frequently assume various tints. In certain varieties of Beech and 

 Beet, they become of a uniform red or copper colour. In some cases, 

 only one of the surfaces of the leaf is coloured, as in many species of 

 Begonia, Saxifraga, Cyclamen, and Tradescantia, in which they are 

 green above and red or brown below ; while in others there is a vari- 

 ation of colour, giving rise to variegation, as in Acuba japonica, 

 Carduus marianus, and Calathea zebrina, where there are yellowish 

 spots, or in many Arums, where they are of a red colour. The whitish 

 or brown spots which occur on leaves, are often produced by thickened 

 cells containing peculiar colouring matter, underlying the chlorophylle 

 cells. In such cases, variegation might be traced to an alteration in 

 the epidermal cells, and the same is true of certain bright colours 

 assumed by the surfaces of some leaves. The juices of many plants 

 are colourless when contained in the vessels, but become milky or 

 coloured by exposure to the ah*. Thus, the sap of (Enanthe crocata 

 becomes yellow, that of Chelidonium orange, that of Madder changes 

 from yellow to red, and that of some Boletuses becomes blue or bluish- 

 green. In some instances, the changes have been prevented by keeping 

 the cut or broken surfaces in nitrogen or hydrogen, or carbonic acid, 

 and thus preventing their exposure to oxygen. It is said, however, that 

 the change of colour in the Madder does not take place in pure oxygen. 



677. The bark, at first green, becomes often of a brown colour from 

 the thickening of the cell-walls, as well as the deposition of brown 

 matter. Similar changes take place in the woody fibres, giving rise 

 to the coloured duramen of many trees, as the Laburnum, Guaiac, 

 Ebony, &c. Such changes, however, depend on chemical actions going 

 on in the interior of stems, and are not due to the direct influence of 



