Chromatic Adaptation. 239- 
are green, and depend on the same rays as plants in air, there is 
found an intermediate zone of brown algae which make up for 
some deficiency of red light by an added power to absorb, to a certain 
extent, green and blue rays. 
These three coloured algal strata, then, tend to possess comple¬ 
mentary colours to the lights in which they are bathed, and thus to 
arrive at the maximum absorptive efficiency for their respective 
habitats. 
There are yet other types of algae which are not pure green in 
colour, the Cyanophyceae or blue-green Algae. These are all minute 
forms and show a great range of colouration, being purple, violet, 
brown, yellow, olive green or blue in different cases, though the 
most usual colour is blue-green. This group possesses at the present 
day a much greater mobility of colouration than the other groups. 
It is, of course, well known that the red, brown and blue-green 
algae all contain chlorophyll in their chromatophores, but that these 
organs contain, in addition, red, brown or blue pigment which 
entirely or largely masks the green colour. These adaptational 
pigments all differ profoundly from chlorophyll in chemical consti¬ 
tution (being usually held to be of a proteid nature) and all are 
soluble in water and diffuse readily out of the dead cell. These 
secondary pigments can be readily discharged by dipping a red, 
brown or blue-green alga in boiling water, after which the chloro¬ 
phyll green stands revealed. 
We now come to the interesting point that as these adapta¬ 
tional pigments are additional and not substitutional, it would 
appear that a red alga snould be a more efficient absorber of light 
than a green one, not only at great depths, but also at the surface. 
Nevertheless the red algae have not replaced the green forms at the 
surface, but are only found there in small numbers. The probable 
explanation is that assimilation at the surface of the sea is limited 
by the amount of C0 2 attainable, as it is on land, and that there¬ 
fore additional power of absorbing light-energy is of no assimilatory 
advantage. The red algae have no advantage over the green at the 
surface, though at a sufficient depth the green alga will be completely 
out-classed. Here then in the case of marine algae we have what 
Engelmann has called “complementary chromatic adaptation” 
arrived at in the course of evolution. 
An extremely interesting case has recently been carefully 
