410 LIFE : OUTLINES OF GENERAL BIOLOGY 



the screen of green pigment in the green leaf means that the orange- 

 red rays of the sunlight are absorbed, and they are the rays that are 

 useful in photosynthesis. It may be mentioned that some "light- 

 filters" are exceedingly delicate in action, blocking out accurately 

 a particular series of wave-lengths, so that sharply edged dark 

 bands are seen when the filtered light is analysed in the spectro- 

 scope. The presence of a particular chemical element in the source 

 of the light may readily be detected in this way. Other filters, 

 however, are somewhat diffuse in their action. It is understood, 

 then, that a coloured substance is one which interferes in some 

 way or other with the whiteness of ordinary light. 



This interference may be due to the physical state oi the coloured 

 substance, as when a finely grated surface, or one from which the 

 light is reflected at two different levels, "interferes" with the light 

 and shows colours, often of an iridescent nature. Everyone knows 

 that there is no pigment in the wall of a soap-bubble, which never- 

 theless shines with all the colours of the rainbow. Similarly, there 

 is no pigment involved in the beautiful iridescence of mother-of- 

 pearl. In such cases we may speak, therefore, of physical or 

 structural coloration. 



But while such structural colour is not uncommon, it is not so 

 common as pigmentary colour, which is due to a property of the 

 molecule of the substance in question to absorb or to reflect only 

 certain parts of the visible spectrum. Such substances, occurring 

 in plants and animals, are called organic pigments, and they may 

 be of the most varied chemical nature. It is not yet possible to 

 suggest a consistent theory connecting particular kinds of chemical 

 composition or structure with the particular colours to which they 

 give rise; yet it is of great interest to inquire into the nature of 

 organic pigments from the chemical side. 



Classification of Colours in Organisms. — Before we pass to 

 the chemistry of organic pigments it will be useful to give examples 

 of the different kinds of coloration that are illustrated by living 

 creatures. (A) Structural or physical coloration, due to the 

 texture or architecture of the surface, may be illustrated by the 

 mother-of-pearl layer in the interior of a mollusc's shell, where 

 the lime is laid down, along with organic cement (conchiolin), in 

 very fine laminae, one over another. If the pearly shell is pounded 

 in a mortar, the result is powdered lime. All the iridescence is gone, 

 for it depended on the laminated structure ; only whiteness is left, 

 and that is due to the almost perfect reflection of the white light 

 from the multitudinous surfaces of the chalky powder. In the 

 same way the beautiful colour of a white Narcissus is due, not to 

 any pigment, but to reflection from very numerous sap-vacuoles 

 among the cells of the perianth: so if we compress the white petal 

 between finger and thumb it becomes transparent. 



