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 hfe 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 



