324 COLOTJKS OF PLANTS. 



similar appearances. Some have said that the luminosity of these fungi, 

 as well as of decaying wood, is increased by exposure to oxygen gas. 

 Some consider it as connected with the absorption of oxygen, being 

 in reality a slow spontaneous combustion ; while, according to others, 

 it is referable to the liberation of phosphorus from some of its com- 

 binations in the plant. 



670. These luminous appearances are said not to be confined to fungi. 

 The younger Linnaeus states, that the flowers of Nasturtium, Orange 

 Lily, and African Marigold, at the end of a hot summer day, give out 

 intermittent light. Mr. Dowden and Mr. James confirmed this by 

 observations on the common Marigold and Papaver pilosum ; while 

 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 disposed to 

 attribute them to optical illusions, depending on a peculiar intermit- 

 tent effect on the retina. Some authors mention the occurrence 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. 



671. Colour is not of much importance in botany as regards classi- 

 fication 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. Mir- 

 bel and Henslow have proposed a nomenclature, which consists in 

 referring all natural colours to certain absolute tints and shades, deter- 

 mined according to fixed laws. Thus, the latter assumes three prim- 

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

 derives all others from admixtures of these in definite proportions. 

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

 and ftir^ov, a measure), or measure of colour, the employment of which 

 would lead to an accurate nomenclature. 



672. It has already been remarked, that the green colour of the 

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

 chlorophylle (^[ 19). This waxy 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 chlorophylle. Light acts by the fixation 

 of carbon. The different rays of the spectrum appear to differ in their 

 power of developing the green colour. Senebier performed experiments 



