5§ 
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 
