We learned in previous paragraphs that the roots of plants take food from the soil, and that a condition necessary for the root to do its work for the plant was the presence of available plant food in sufficient quantities.

What is plant food?  For answer let us go to the plant and ask it what it is made of.

=Experiment.=—­Take some newly ripened cotton or cotton wadding, a tree branch, a cornstalk, and some straw or grass.  Pull the cotton apart, then twist some of it and pull apart; in turn break the branch, the cornstalk and the straw.  The cotton does not pull apart readily nor do the others break easily; this is because they all contain long, tough fibres.  These fibres are called woody fibre or cellulose.  The cotton fibre is nearly pure cellulose.

=Experiment.=—­Get together some slices of white potato, sweet potato, parsnip, broken kernels of corn, wheat and oats, a piece of laundry starch and some tincture of iodine diluted to about the color of weak tea.  Rub a few drops of the iodine on the cut surfaces of the potatoes, parsnip, and the broken surfaces of the grains.  Notice that it turns them purple.  Now drop a drop of the iodine on the laundry starch.  It turns that purple also.  This experiment tells us that plants contain starch.

=Experiment.=—­Chew a piece of sorghum cane, sugar cane, cornstalk, beet root, turnip root, apple or cabbage.  They all taste sweet and must therefore contain sugar.

Examine a number of peach and cherry trees.  You will find on the trunk and branches more or less of a sticky substance called gum.

=Experiment.=—­Crush on paper seeds of cotton, castor-oil bean, peanuts, Brazil nuts, hickory nuts, butternuts, etc.  They make grease spots; they contain fat and oil.

=Experiment.=—­Chew whole grains of wheat and find a gummy mucilaginous substance 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.