ACCUMULATION OF ENERGY 95 



PIGMENTS 



Photosynthesis is related to the presence of the green 

 colouring matter called chlorophyll. Therefore it is necessary 

 to examine the relation of pigments to light, after which we 

 can return to the subject of photosynthesis. 



The colour of an object depends upon the absorption and 

 reflection of the light of different wave lengths. The study 

 of coloration has a much wider scope than the chemical 

 discussion of pigments, as some colours are due to structural 

 arrangements whereby light is reflected in such a way that 

 interference of one wave with another produces colours. 

 The significance of coloration is a biological problem of great 

 interest. 



White light can be decomposed by means of prisms into a 

 series of coloured lights forming the spectrum. The instrument 

 by which this is done is called a spectroscope, and it is used 

 for the study of chlorophyll and other pigments. 



The spectrum of sunlight shows narrow dark lines parallel 

 to the coloured bands. These lines are called Frauenhofer 

 lines, and they are useful for measurement of other lines : 

 they are due to absorption of the light of certain wave lengths 

 by the vapour, in the sun, of the same elements which furnish 

 those wave lengths when incandescent. 



Newton described seven colours in the spectrum, but most 

 people can recognise only six. 



Absorption of light may take the form of diffuse absorption 

 at the ends of the spectrum, or of dark bands crossing the 

 coloured area. Absorption occurs also in the invisible 

 spectrum, either in the infra red or ultra violet region : the 

 former being recognised by its heating effect and the latter 

 by its effect on the photographic plate. 



It is only the energy of light that is absorbed that can be 

 used to add energy to the system. 



Colours in plants and animals are useful, in two ways, in 

 addition to the process of photosynthesis, namely, for protec- 

 tion and for sexual selection. It is characteristic of the 

 economical working of biological processes that waste products 

 are sometimes used for purposes of coloration.* 



In plants the colours are largely contained in the specialised 

 leaves which form the flowers and they are therefore of 

 sexual importance. The colours of flowers are developed by a 

 process of oxidation. By studying extracts of flowers it is 



* F. G. Hopkins, Phil. Trans., 1895, B. vol, 186, part II, p. 661. 



