16 COLOUR IN NATURE chap. 
are already familiar to most people. It is more to 
our purpose to note the organisms among which 
it occurs, and the special parts with which it is 
associated. 
To begin with the simplest organisms—the fact 
that many decaying substances shine in the dark 
has long been known, and modern bacteriologists 
have been able to isolate the specific micro-organisms 
which possess, under certain conditions, this property 
of evolving light. Just as in the case of pigment 
production, so also the phosphorescence depends upon 
the nature of the organism and the character of its 
surroundings. Thus Photo-bacterium phosphorescens 
evolves light during the fermentation of sugar if free 
oxygen be present and the temperature be favour¬ 
able (3°-3 5 ° C.). 
Among the Protozoa, phosphorescent forms are 
most noticeably represented by the genus Noctiluca , 
but some Radiolarians, such as Thalassicola , are also 
luminous. In Noctiluca the phosphorescence is said 
by Allman to be produced in the cortical layer of 
protoplasm. 
Among multicellular plants phosphorescence is 
apparently confined to the fungi. It is an old 
controversy whether it does or does not occur in 
flowering plants, but most authorities answer the 
question in the negative. In the fungi the 
luminosity is marked in several species of Agaricus. 
The phenomenon is only manifest in the presence of 
free oxygen, is associated with oxidative processes, 
and is dependent upon the temperature. The light 
is said to be usually of blue or greenish colour, but 
in some cases is quite white. 
