water. It is found
in a state of considerable purity in gum tragacanth and some other gums.
Its composition is not known with absolute certainty, but it is either
C_{24}H_{19}O_{19}, or C_{12}H_{10}O_{10}; and in the latter case it
must be identical with starch and gum.

It will be observed that all the substances belonging to this class are
very closely related in chemical composition, some of them, as starch
and gum, though easily distinguished by their properties, being
identical in constitution, while others only differ in the quantity of
water, or of its elements which they contain. In fact, they may all be
considered as compounds of carbon and water, and their relations are,
perhaps, more distinctly seen when their formulae are written so as to
show this, as is done in the following table, in the second column of
which those containing twelve equivalents of carbon are doubled, so as
to make them comparable with cellulose:--

Water.
Grape sugar, C_{12}H_{12}O_{12} C_{24}H_{24}O_{24} C_{24} + 24
Cane sugar, C_{12}H_{11}O_{11} C_{24}H_{22}O_{22} C_{24} + 22
Cellulose, C_{24}H_{21}O_{21} C_{24}H_{21}O_{21} C_{24} + 21
Inuline, C_{24}H_{21}O_{21} C_{24}H_{21}O_{21} C_{24} + 21
Starch, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
Dextrine, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
Gum, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20
Mucilage, C_{12}H_{10}O_{10} C_{24}H_{20}O_{20} C_{24} + 20

The relation between these substances being so close, it is not
difficult to understand how one may be converted into another by the
addition or subtraction of water. Thus, cellulose has only to absorb an
equivalent of water to become grape sugar, or to lose an equivalent in
order to be converted into starch, and we shall afterwards see that
such changes do actually occur in the plant during the process of
germination.

_Pectine and Pectic Acid._--These substances are met with in many fruits
and roots, as, for instance, in the apple, the carrot, and the turnip.
They differ from the starch group in containing more oxygen than is
required to form water along with their hydrogen; but their exact
composition is still uncertain, and they undergo numerous changes during
the ripening of the fruit.

_2d. Oily or Fatty Matters._--The oily constituents of plants form a
rather extensive gr