Most housekeepers have at least a few house plants and many have gardens which occupy part of the time each day. All foods are directly or indirectly produced by plants and it is well to consider also what food these living things require in their turn.
Plants are able to take from the materials forming the crust of the earth and from the air surrounding them all that they need for their life. The leaves of the plants, because of the green substance called chlorophyl, have the power of decomposing carbon dioxide gas in a such a way that plants make use of the carbon and breathe out oxygen. Fig. 31. This separation is very difficult to make in the laboratory. The energy of sunlight is utilized by the plant for this work, for the action does not take place in darkness. In this way plants return to the air the oxygen so necessary for animal life and are themselves fed in part by the useless and even harmful gas exhaled by animals.
The soil on which the plant grows furnishes the mineral matter needed. When plant tissues are burned, these mineral substances remain as ashes. When the ashes of plants are analyzed, they are found to consist of potash, soda, iron, and lime in the form of phosphates, sulphates, and silicates. Some of these substances are present in the soil in inexhaustible quantities, but others are less abundant and unless the soil be fertilized from time to time, the plant soon uses them up. These less abundant substances are phosphates, potash, and nitrogen.
The lover of house plants has long resorted to various expedients for feeding them, and many plant foods are now sold and in common use. In using these for manuring potted plants, care must be taken not to use too much, since strong solutions of them are likely to corrode the roots and kill the plants.
Although nitrogen is a very abundant element, forming as has been said, four-fifths of the air, yet it is comparatively rare in forms which are of use to plants. As a rule plants cannot take it from the air and therefore require soluble compounds of nitrogen for food. One of the most important of these is ammonia. This is formed when organic substances decay, its odor being very noticeable about stables. Its action with acids was described in the pages about cleaning and it was explained how it unites with acids to form salts, usually soluble. Sulphate of ammonia is the form used in agriculture. A very little ammonia in the water used on house plants is a good thing for them.
It has been seen that plants by aid of sunlight breathe in carbon dioxide and breathe out oxygen gas. In addition to this, they also breathe as animals do, to a slight extent, taking in oxygen and breathing out carbon dioxide. This action is more pronounced in darkness.
The wonderful principle called conservation is illustrated by what we know of plant life. Plants in growing store up energy derived from the heat and light of the sun. When they decay, or are burned, or are eaten by animals, exactly the same amount of energy is set free and changed into a new form, and just as much carbon dioxide as the plant breathed in, is given back to the air. A plant which was many years in growing may be consumed in an hour or may decay slowly for years. In either case the same total amount of energy is set free, fast or slowly. This energy is most apparent as heat. In the growth and destruction of the plant both energy and matter have been transformed, but neither energy nor matter has been made or lost - it has merely taken on a new appearance. When animals feed on plants they transform the energy of sunlight which is stored up in the plant into energy of vitality. In this sense man and all animals are "children of the sun."
Fig. 29. A Battery of Cells Connected In Series
Fig. 30. A Dry Cell