392' COLOUES OF PLANTS. 



become yellow in consequence of the great alterability of the blue 

 colouring principle, unless the latter is rendered more stable by union 

 with an acid which reddens it. In this case the leaf becomes red. 

 In Selaginella mutabilis the colouring matter becomes aggregated in 

 balls at certain times, while at other times it is diffused. This accounts 

 for the change of colour in bright and in diffused light. In this plant 

 the leaves are pale mUk-white in the evening or in cloudy weather, 

 and bright green in sunshine. 



Sorby classifies the colouring matter of leaves as follows : — 



1. Chlorophyll {x!^(jip6s, green, 0i5X\op, a leaf), a green colouring matter, rarely found 



pure, insoluble in water, but soluble in alcohol or bisulphide of carbon. 

 Spectral tint more or less green. 



2. Xanthophyll {^aMs, yellow), yellow group, insoluble in water, soluble in alco- 



hol and bisulphide of carbon. General spectral colour clear yeUow or orange. 



3. Brythrophyll {ipvBpSs, red), red colouring matter, soluble in water and aqueous 



alcohol, insoluble in bisulphide of carbon. Strong absorption in the green 

 part of the spectrum. 



4. Chrysophyll (xjoucis, gold), golden yellow group, soluble in water and in aqueous 



alcohol, insoluble in bisulphide of carbon. Varying spectra. 

 5.' Phaiophyll {<paios, brown), group comprising various browns, soluble in watet, 

 but not in bisulphide of carbon. No weU-deflned absorption-bands in the 

 spectrum. 



Groups of colours as leaves advance to decay : — 



Complete vitality and growth j chloroDhvU ) ( ^°^^ °^ ^^^^ bright green. 

 ErvthrophvU ^'"'^ °^ ^^^^ green-brown. 

 Low vitality and change . . j XanthophvU I ( ^°^^ or less red-scarlet. 



( Phaiophvll ( 1 ^°^'^ °^ ^^^^ bright orange-brown. 

 Death and decomposition . j g;umiis 1 ■''^^^ °^ '^"^^ ^^ brown. 



{Nature, Jan. 19, 1871, p. 237.) 



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

 principle, chromogen (p^gS/ia, colour, and yeviidto, I generate,) consisting 

 of two separate principles, one of which forms a red compound 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, iu 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, Saxi- 

 fraga, Cyclamen, and Tradescantia, in which they are green above 

 and red or brown below ; while in others there is a variation of colour, 

 giving rise to variegation, as in Aucuba japonica, Oarduus marianus, 



