e below) can be obtained by hydrolysis.
Chlorophyll _a_ is blue-black, is easily soluble in most organic solvents,
and when saponified by alcoholic potash gives a transient pure yellow
color. Chlorophyll _b_ is dark green, is somewhat less soluble than the
other form, and when saponified by potash gives a transient brilliant red.

=Amorphous and Crystalline Chlorophyll.=--When the chlorophyll of plants is
extracted by alcohol and the alcoholic extract evaporated nearly to
dryness, beautiful dark green crystals are obtained. Willstaetter has shown,
however, that in these crystallized forms the ethyl group (from the ethyl
alcohol used) has replaced the phytyl group (see below) which is present in
the pigments as they exist in the plant tissues; and that, when extracted
by other solvents than alcohol, the pigments may be obtained in the
amorphous forms in which they exist in the plant.

This change from amorphous to crystalline compounds may be understood from
the preliminary statement that the chlorophylls are esters of tri-basic
acids, in which one acid hydrogen is replaced by the methyl (CH_{3}) group
and a second by the phytyl (C_{20}H_{39}, from phytol, or phytyl alcohol,
C_{20}H_{39}OH) group. When treated with ethyl alcohol (C_{2}H_{5}OH) for
the purpose of extracting the pigments, the ethyl (C_{2}H_{5}) group
replaces the phytyl group, thus yielding a methyl-ethyl ester, and these
esters are the crystalline forms of the chlorophylls. This replacement is
made possible through the action on the original pigment in the tissues of
an enzyme, _chlorophyllase_, which is also present in the tissues, which
splits off the phytyl group, forming phytyl alcohol, and leaving a free
COOH group in the pigment, with which the alcohol used in the extraction
forms the ethyl ester (see Chapter IX for a discussion of the formation and
hydrolysis of esters).

While the chlorophylls are tri-basic acids, only two of the acid COOH
groups actually function in ester-formation. The third acid group seems not
to exist as a free acid group; but in chlorophyll _a_, it is in what is
known as the "lactam" arrangement, represented by the --CONH-- group, and
in chlorophyll _b_, it is probably in the "lactone" arrangement,
represented by the --COO-- group; the two bonds in each case being attached
to different structural units in the molecule (see page 106).

The change from amorphous to crystalline forms may be represented by the
following formul