[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. | they are made. | which plants | | | obtain them. | -------------------+---------------------+----------------+ Cellulose or | | | woody fibre | Carbon | Air | Starch |---------------------+----------------+ Sugar | | | Gum | Oxygen | Water | Fat and Oil | Hydrogen | | -------------------+---------------------+----------------+ | Carbon | Air | +---------------------+----------------+ Albuminoids | Oxygen | Water | | Hydrogen | | +---------------------+----------------+ | _Nitrogen_ | | | Sulphur | | | Phosphorus | | -------------------+---------------------| Soil + | _Phosphorus_ | | | _Potassium_ | | Ashes | _Calcium_ | | | Magnesium | | | Iron | | -------------------+---------------------+----------------+ Water | Oxygen | Soil | | Hydrogen | | -----------------------------------------+----------------+
