._--These substances are
compounds of carbon, hydrogen, and oxygen, and all possess a certain
degree of similarity in composition, the quantities of hydrogen and
oxygen they contain being always in the proportion required to form
water, so that they may be considered as compounds of carbon and water;
not that it can be asserted that they actually do contain water, as
such, for of that there is no evidence, but only that its elements are
present in the proportion to form it.

_Cellulose._--This substance forms the fundamental part of all plants.
It is the principal constituent of woody fibre, and is found in a state
of purity in the fibre of cotton and flax, and in the pith of plants;
but in wood it is generally contaminated with another substance, which
has received the name incrusting matter, because it is deposited in and
around the cells of which the plant is in part composed. Cellulose is
insoluble in all menstrua, but, when boiled for a long time with
sulphuric acid, is converted into a substance called dextrine. Cellulose
consists of--

From pith of Elder-tree. Spongioles of roots.

Carbon 43.37 43.00
Hydrogen 6.04 6.18
Oxygen 50.59 50.82
------- -------
100.00 100.00

It is represented chemically by the formula, C_{24}H_{21}O_{21}, which
shows it to be a compound of 24 atoms of carbon with 21 of hydrogen and
21 of oxygen.

_Incrusting matter._--Large quantities of this substance enter into the
composition of all plants. Of its chemical nature little is known, as it
cannot be obtained separate from cellulose, but it is analogous to that
substance in its composition, and probably contains hydrogen and oxygen
in the proportion to form water.

_Starch._--Starch is one of the most abundant constituents of plants,
and is found in most seeds, as those of the cereals and the leguminous
plants; in the tubers of the potatoe, the bulbs of tulips, &c. &c. It is
obtained by placing a quantity of wheat flour in a bag, and kneading it
under a gentle stream of water. When the water is allowed to stand, it
deposits the starch as a fine white powder, which, when examined by the
microscope, is found to be composed of minute grains, formed of
concentric layers deposited on one another. These grains vary
considerably in size and structure in different plants; but in th