34 
COLOUR IN NATURE 
CHAP. 
again in further detail when we come to consider 
the colours of plants. 
So far as is yet certainly known, chlorophyll is 
the only pigment by means of which organisms can 
avail themselves of the carbon of the atmosphere, 
though it is quite possible that some other pigments 
may yet be shown to possess this property. The 
suggestion has been made in the case of the bright 
pigments of coral polypes, as well as of various 
other pigments, notably of the purple pigment of 
Beggiatoa roscopersicina (see Chap. III.). 
Pigments fulfilling the function of haemoglobin 
are perhaps fairly numerous, but there is some doubt 
connected with many of these so-called respiratory 
pigments. Of those certainly respiratory, haemo- 
cyanin is by far the most important. 
The question as to whether any other pigments 
besides those of the nature of chlorophyll and the 
respiratory pigments are of direct physiological 
importance, is a somewhat difficult one. According 
to many, some pigments, such as the colouring 
matter of the Red Algae and the dark pigments of 
many animals, are of great importance in protecting 
delicate tissues from the injurious effects of certain 
of the rays of white light. 
Again, the pigment described by Dr. MacMunn as 
enterochlorophyll, which is so common in connection 
with the alimentary tract in many Invertebrates, is 
supposed by some to be of importance in digestion 
or assimilation. There are other suggestions of the 
same kind, but there is as yet little certainty, the 
functions of the large majority of pigments being 
quite unknown. 
