These differences are of great importance to the practical farmer, as by understanding what kind of plants use the most of one ingredient, and what kind requires another in large proportion, he can regulate his crops so as to prevent his soil from being exhausted more in one ingredient than in the others, and can also manure his land with reference to the crop which he intends to grow. The tables of analyses in the fifth section will point out these differences accurately. FOOTNOTES: [J] This pectic acid gelatinizes food in the stomach, and thus renders it more digestible. [K] See Johnston's Elements, page 41. [L] Sifted through a fine cloth called a bolting cloth. CHAPTER VIII. RECAPITULATION. We have now learned as much about the plant as is required for our immediate uses, and we will carefully reconsider the various points with a view to fixing them permanently in the mind. Plants are composed of _organic_ and _inorganic_ matter. [What is organic matter? Inorganic? Of what does organic matter consist? Inorganic? How do plants obtain their organic food? How their inorganic? How is ammonia supplied? Carbonic acid?] Organic matter is that which burns away in the fire. Inorganic matter is the ash left after burning. The organic matter of plants consists of three gases, oxygen, hydrogen and nitrogen, and one solid substance carbon (or charcoal). The inorganic matter of plants consists of potash, soda, lime, magnesia, sulphuric acid, phosphoric acid, chlorine, silica, oxide of iron, and oxide of manganese. Plants obtain their organic food as follows:--Oxygen and hydrogen from water, nitrogen from some compound containing nitrogen (chiefly from ammonia), and carbon from the atmosphere where it exists as carbonic acid--a gas. They obtain their inorganic food from the soil. The water which supplies oxygen and hydrogen to plants is readily obtained without the assistance of manures. Ammonia is obtained from the atmosphere, by being absorbed by rain and carried into the soil, and it enters plants through their roots. It may be artificially supplied in the form of animal manure with profit. Carbonic acid is absorbed from the atmosphere by leaves, and decomposed in the green parts of plants under the influence of daylight; the carbon is retained, and the oxygen is returned to the atmosphere. [When plants are destroyed by combustion or decay, what becomes of their constituents? How does the inorganic matter enter the plant? Are the alkalies soluble in their pure forms? Which one of them is injurious when too largely present? How may sulphuric acid be supplied? Is phosphoric acid important? How must silica be treated? From what source may we obtain chlorine?] When plants are destroyed by decay, or burning, their organic constituents pass away as water, ammonia, carbonic acid, etc., ready again to be taken up by other plants.