i THE COLOURS OF ORGANISMS 7 
are known in the arts as pigments, but to the 
biologist the term includes only substances which 
are produced by the activities of plants or animals. 
The colour of the rose, for example, is due to a red 
pigment present in the cells of the petals, which can 
be extracted from those cells. This is the simplest 
form of colour-production, and is the one which 
commonly occurs in plants. Colours so produced 
are called pigmental colours and can be recognised 
by the following characters. Pigmental colours are 
those produced by pigments, or substances of definite 
chemical composition, which can by appropriate re¬ 
agents be extracted from the coloured tissues, and 
which react to light in the same way whether they 
are within the tissues or outside of them. Tissues 
or organisms showing only pigmental colours never 
have a surface gloss, and the colour is not altered by 
immersion in any medium which does not directly 
attack the pigment. 
Although the great majority of the colours of 
plants are produced in this simple way, yet even 
among them we have indications of that other kind 
of colour-production which reaches its climax of 
splendour in birds and butterflies. The gorgeous 
tints of the humming-birds, which during life change 
with every movement, are not produced by the 
dyeing of the feathers with pigment, but are 
phenomena of the same order as the colours of 
the gems after which some of the birds are named. 
We all know that the colours of the opal and of 
many minerals are not due simply to a prime 
property of the substance concerned, but are optical 
effects dependent upon various external conditions 
