extracted from the tissues is by rupturing the cells, by
grinding with sharp sand, etc., after which the pigments can readily be
dissolved out by water.

=Phycoerythrin= is the red pigment. It is a colloidal, nitrogenous
substance, allied to the proteins (see Chapter XIII) but not a true protein
compound. Hydrolysis by acids indicates that it contains leucine and
tyrosine, two amino-acids which are constituents of proteins, along with
other bodies of unknown composition.

The colloidal solution of phycoerythrin in water has a brilliant rose-red
color, with an orange fluorescence. It readily sets to a gel (see Chapter
XV), so that the solution is almost impossible to filter. On this account,
purified solutions of this pigment are very difficult to secure, and no
satisfactory analysis to indicate its composition has yet been obtained.

Actinically, it is a complementary pigment to chlorophyll, that is, it
absorbs the blue and green rays and permits the passage of light which is
of the wave length that is absorbed by chlorophyll.

=Phycophaein.=--Still less is known of the composition of this pigment than
of that of phycoerythrin. It is the characteristic pigment of brown
seaweeds. It is supposed to exist in the cells of algae, chiefly as a
colorless chromogen, which becomes first yellow and then brown on exposure
to air. Associated with it are other pigments, which have been variously
reported as carotin, phycoxanthin, etc.

THE ANTHOCYANS

These are a group of pigments of red, blue, or violet color, which occur in
the flowers, fruits, or leaves of many species of plants. They are
essentially ornamental pigments, and constitute a large proportion of the
brilliant colors of flowers, etc. They occur not only dissolved in the
cell-sap, but also as deposits of definite crystals or amorphous compounds
in the cell protoplasm.

They are all glucosides. When the anthocyans are hydrolyzed, the sugar
molecules are split off and the characteristic hydroxy-derivatives of the
three-ring anthocyan nucleus (figured on page 83), known as
"anthocyanidins," remain. These anthocyanidins are themselves pigments.
They have been shown to be all derivatives of the anthocyan nucleus. The
oxygen atom in this nucleus is very strongly basic and exhibits its
quadrivalent property by forming stable salts by direct addition of acid
radicles. The variation of color of the anthocyanins has been explained by
Willst