called wheat gum, or wet a pint of wheat flour
to a stiff dough, let it stand about an hour, and then wash the starch
out of it by kneading it under a stream of running water or in a pan
of water, changing the water frequently. The result will be a tough,
yellowish gray, elastic mass called gluten. This is the same as the
wheat gum and is called an albuminoid because it contains nitrogen and
is like albumen, a substance like the white of an egg.

If we crush or grate some potatoes or cabbage leaves to a pulp and
separate the juice, then heat the clear juice, a substance will
separate in a flaky form and settle to the bottom of the liquid. This
is vegetable albumen.

[Illustration: FIG. 34.
Soy-bean roots. Showing nodules of tubercles, the homes of
nitrogen-fixing bacteria.]

[Illustration: FIG. 35.
Garden-pea roots, showing tubercles or nodules, the homes of
nitrogen-fixing bacteria.]

=Experiment.=--Crush the leaves or stems of several growing plants and
notice that the crushed and exposed parts are moist. In a potato or an
apple we find a great deal of moisture. Plants then are partly made of
water. In fact growing plants are from 65 to 95 per cent. water.

=Experiment.=--Expose a plant or part of a plant to heat; the water is
driven off and there remains a dry portion. Heat the dry part to a
high degree and it burns; part passes into the air as smoke and part
remains behind as ashes.

We have found then the following substances in plants: Woody fibre or
cellulose, starch, sugar, gum, fats and oils, albuminoids, water,
ashes. Aside from these are found certain coloring matters, certain
acids and other matters which give taste, flavor, and poisonous
qualities to fruits and vegetables. More or less of all these
substances are found in all plants. Now these are all compound
substances. That is, they can all be broken down into simpler
substances, and with the exception of the water and the ashes, the
plants do not take them directly from the soil.

The chemists tell us that these substances are composed of certain
chemical elements, some of which the plant obtains from the air, some
from the soil and some from water.

The following table gives the substances found in plants, the elements
of which they are composed, and the sources from which the plants
obtain them:

----------------------------------------------------------+
Substances found | Elements of which | Sources from |
in plants. |