58 COLOUR IN NATURE chap. 



Algae are typically deep-water forms, and it is well 

 known that water absorbs the various rays of light 

 unequally, so that the yellow and red rays do not 

 penetrate to great depths, but are stopped long before 

 the blue and violet. Engelmann (1883) experimented 

 upon the effect of the different rays of the spectrum 

 upon the process of assimilation in various plants, 

 and found that while green plants, generally speaking, 

 assimilate best in red light, the Red Algae assimilate 

 best in blue light, and can therefore live at depths 

 impossible to other plants. This does not of course 

 assist us as to the meaning of the phycoerythrin, 

 but Kerner has supplemented Engelmann's observa- 

 tions by the suggestion that the red pigment absorbs 

 the blue and violet rays and converts them into red 

 rays, so that according to him the presence of the 

 red pigment is absolutely necessary to these Algae. 

 The suggestion is perhaps chiefly of interest because 

 it is one of a great number which have been made 

 lately as to the effect of pigments on the incident 

 rays of light, and which assign a physiological im- 

 portance to pigments hitherto regarded as useless. 

 Nothing is known as to the chemical affinities of 

 phycoerythrin. It is possibly of proteid nature, 

 and like numerous other pigments has been, on ex- 

 tremely doubtful evidence, accredited with a respira- 

 tory function. 



We do not propose to go into further detail on 

 the subject of pigments associated with chlorophyll, 

 but may mention one or two cases in the Algae of 

 that replacement of chlorophyll by other pigments 

 in special regions which is so common in the higher 

 plants. In many of the unicellular Algae, as in many 



