COLOURS OF PLANTS. 327 



the air. The colour of wood, however, is generally deepened when 

 exposed to the atmosphere. 



678. The red, blue, and yellow colours of flowers depend on fluid or 

 semifluid matters contained in superficial cells, which can be detached with 

 the cuticle. This coloured cellular tissue is called by Nourse the Rete, 

 and lies immediately below the epidermis. In this respect these colours 

 differ from the green colour hi the leaf which is confined to the central 

 cells, and which, as already stated, owes its origin to a granular matter of 

 a peculiar nature. In petals, different cells frequently contain different 

 kinds of colouring matter, thus giving rise to variegation. By the 

 juxta-position and mechanical mixture of various cells, different tints 

 are produced ; and the colours are also modified by the nature of the 

 cuticle through which they are seen. In the interior of petals, the 

 colour is generally more or less yellow, but it is modified when seen 

 through superficial cells. Along with the colouring matter, there is a 

 colourless substance present, the relative quantity of which varies, and 

 hence the colour may be deeper or fainter. In flowers as well as in 

 leaves, the colours appear to depend on the action, of light. It has been 

 said, however, that the powerful action of solar light, in some cases, 

 tends to decolorize flowers. Hence, tulips are screened by floriculturists 

 from the direct rays of the sun. The leaves of herbaceous plants 

 also, when exposed to the direct rays of the sun, do not acquire so 

 deep a green as when they are subjected merely to a bright daylight. 



679. The colours of flowers have been arranged in two series : 

 1st. The xanihic (^otvSog, yellow) or yellow; and 2d. The cyanic (xt/oez/oj, 

 blue) or blue; and it has been shown that plants in general may be 

 referred to one or other of these series, while red is common to both 

 series, and green, as composed of blue and yellow, is intermediate be- 

 tween them. White is considered as depending on absence or extreme 

 dilution of the colouring principles, whue brown or black depends on 

 their accumulation or concentration. Even in white flowers there will 

 be seen a slight admixture of a yellowish or bluish tint. 



Green, 



\ 



Greenish-blue. 

 Blue. 



Cyanic j Violet-blue, 

 series. 1 Violet. 



Violet-red. 

 [Bed. 



Yellowish-green. ~) 



Yellow. 



Orange-yellow. I Xanthic 



Orange. ( series. 



Orange-red. 



Bed. 



680. Some starting from greenness, as a state of equilibrium between 

 the two series, pass through the blue and violet to red, by a process of 

 oxidation, while the transition from red to orange and yellow has been 

 traced to deoxidation. As illustrations of the cyanic series may be 

 mentioned, all, or nearly all, the species of Campanula, Phlox, Epilo- 

 bium, Hyacinth, Geranium, Anagallis ; of the xanthic series, Cactus, 



