390 COLOURS OF PLANTS. 



other observers have noticed the phenomena in the Sun-flower, French 

 Marigold, species of (Enothera, and Arum. It is to be remarked 

 that the flowers said to be thus luminous are all of a more or less 

 orange colour, and that the phenomenon takes place in still warm 

 summer evenings, towards twilight. Hence, Professor AlLman is dis- 

 posed to attribute them to optical illusions, depending on a peculiar 

 iutermittent effect on the retina. Some authors mention the occur- 

 rence of luminous sap in plants with milky juices, as the Euphorbia 

 phosphorea of Brazil. A rhizome of an endogenous plant from India, 

 is said, when moistened, to acquire a phosphorescent appearance, and 

 to lose this property when dry. 



4. — Colours of Plants. 



Colour is not of much importance in Botany as regards classiSca- 

 tion and arrangement. It is chiefly in the case of Fungi that it is 

 employed as a means of diagnosis. Perhaps the want of an accurate 

 nomenclature of colours in Botany may have in part led to this. 

 Mirbel and Henslow have proposed a nomenclature, which consists in 

 referring all natural colours to certain absolute tints and shades, 

 determined according to fixed rules. Thus, the latter assumes three 

 primaries, as red, blue, and yellow, which together give white light, 

 and derives all others from admixtures o^ these in definite proportions. 

 On this principle he has constructed a chromatometer (^^Sj/^a, colour, 

 and //.sTgoii, a measure), or measure of colour, the employment of which 

 would lead to an accurate nomenclature. 



It has already been remarked that the green colour of the leaves, 

 young bark, calyx, and carpels, depends on the presence of chlorophyll 

 (p. 12). This substance is contained in the deep cells or mesophyllum 

 of leaves, and depends on the action of light for its elaboration. 

 When leaves are grown in darkness they become colourless from the 

 absence of chlorophyll. Light acts by the fixation of carbon. The 

 different rays of the spectrum appear to vary in their power of de- 

 veloping the green colour. Senebier performed experiments on the 

 subject, by making the light pass through coloured media, and he 

 was led to the conclusion that while the yeUow rays had the greatest 

 effect on the growth of the plant, the blue and chemical rays were 

 those chiefly concerned in the production of the green colour. Hunt 

 seems to agree with Senebier. Other experimenters, however, as Mor- 

 ren, Daubeny, Draper, and Gardner, think the yellow rays are the 

 most active in producing the green colour. The following table shows 

 the result of some of Gardner's experiments. The rays are denomi- 

 nated active or inactive in relation to their power of producing a green 

 colour, and the figures under each of them show their power in this 



