4 SEAWEEDS 
The first observation commonly made by a student 
of seaweeds is of the variation of their colours. The 
green hue that prevails throughout land vegetation, 
except in the colours of flowers and the bark of 
trees, is varied in the case of seaweeds with olive- 
brown, and red forms. An artificial classification of 
them according to their colours leads to the striking 
result that it nearly coincides with the natural classi- 
fication of them according to their structure and 
development. Such an artificial classification be- 
came firmly established, and has left its mark on the 
names of the natural primary divisions or sub-classes 
of Algee, viz. the Khodophyceee or Red Seaweeds; the 
Phoeophycee, or olive-brown; the Chlorophycee or 
green; and the Cyanophycee or blue-green. A 
simple experiment proves that fundamentally they 
are all green, and that the red colouring matter 
phycoerythrine, the brown phycopheine, the yellowish- 
brown phycoxanthine, and the blue phycocyanine are 
each something added to the chlorophyll or leaf-green 
that characterises vegetation in general, and by 
virtue of which plants form the organic substances 
necessary for their nutrition. These additional colour- 
ing matters can be extracted by fresh water, leaving 
the previously red, olive, &c. plants green, and they 
differ from the green colour in this respect, since it 
is insoluble in water. Though there occur excep- 
tionally a few red forms, numerous blue-green, and 
(in the diatoms only) many brown forms in fresh- 
waters, there still remains the broad fact that these 
colouring matters are characteristic of seaweeds, and 
it is in the conditions of plant-life in the waters of 
