845. The object of manuring is mainly to increase the quantity of organic matter, or to restore to the soil those qualities which have been taken away by the crops. By various amendments (as gypsum, lime, charcoal) ammonia is strongly attracted from the air and yielded again to vegetation. Marl promotes the decomposition of the soil, and ashes add to the potassa - a substance which also exists naturally in soils, being derived from the decomposition of the rocks which contain it, as granite, clay-slate, basalt, etc.

846. Bone manure is rich in the phosphates indispensable in the formation of albumine, gluten, and other blood-making qualities of fruits. The mineral phosphate of lime, bone-chalk, etc., are of the same nature.

847. Guano is a manure whose great value depends upon its abundant nitrates and ammoniacal salts. It is the excrement of sea-fowl which has for ages accumulated in vast deposits on certain coasts and islands of South America and Africa.

848. Fallow ground. Soils are often improved by lying fallow for a season, thus allowing time to form, by decomposition, a fresh supply of that particular ingredient which had been exhausted by previous crops. On the same principle is explained the beneficial effects of a rotation of such crops as require different mineral substances in their composition.

849. Light and heat. After all these materials have been supplied to the plant, still two other agents are requisite, without which the great work of vegetation will not go on. These life-giving principles are light and heat, both of which emanate in floods from the sun. Under their influence the raw material is received into the vessels of the plant and assimilated to its own substance - a process which can be fully comprehended only by Him whose power is adequate to carry it on.

850. Digestion. Under the influence of solar light and a temperature above the freezing point, water holding various earths in solution is imbibed by the roots and raised into the tissues of the stem, dissolving as it passes small portions of gum or sugar previously deposited there. In this state it is crude sap. Passing on it enters the leaves, and is there subjected to the action of the chlorophylle (§ 657) which chiefly constitutes the apparatus of digestion. Here it is concentrated by transpiration, sending off quantities of pure water. Meanwhile the leaves are imbibing carbonic acid from the air, decomposing it, retaining the carbon, and returning pure oxygen. Thus elaborated, the sap is now called

851. The proper juice, consisting evidently of carbon and water, with a little nitrogen and minute portions of the mineral substances mentioned above. From this, the vital fluid, are elaborated the building material of the vegetable fabric, and all its various products and secretions.

852. Protein, or protoplasm, the substance of the primordial utricle, analogous in composition to the living tissues of animals, and containing nitrogen, is organized, first of all, from this vital fluid. Cellulose (or lignin) next, the peculiar principle of vegetable tissue, whether cellulal, vasoular, or woody, consisting of carbon with the exact elements of water, viz., C12 H10 O10. Then, through the action of light, chlo-rophylle springs into being, clothing the plant in living green Meanwhile

853. Gum, starch and sugar, nutritive products common to all plants, are also developed from the proper juice - not all to be immediately employed in building up the tissues, but mostly to be stowed away in reserve for future use. Such deposits are made in the root of the beet, tuber of the potato, and in the fruit generally. These three products, with cellulose, are all composed of carbon and the elements of water, often in identical proportions; thus cane sugar is C12 H12 O12; grape sugar, C12 H11 O11; gum, C12 H10 O10; starch, C12 H10 O10; cellulose, C12 H10 O10.

854 Sugar is sometimes produced directly from the proper juice, as in the root of beet, stalk of maize, and sugar-cane; but oftener, during germination, from the starch deposited in the seed. Its composition, as seen above, differs from that of starch only in containing a larger proportion of the elements of water or (what is the same) a smaller proportion of carbon. As starch is insoluble, its transformation into soluble gum or sugar is needful to render it available for the nutrition of the growing embryo.

855. The facility with which these FIVE general products are con-verted into each other, both in the growing plant and in the laboratory of the chemist, is accounted for by the similarity of their chemical condition. Thus starch, gum and cellulose may reconvert merely by some change in the arrangement of their constituent atoms, or they may become sugar by the addition of one or two atoms of water.

856. Among the numerous secretions of plants, which our limits forbid us to consider, are the vegetable acids containing more oxygen proportionately than exists in water; the oily acids, resins and oils, containing less oxygen than exists in water, or none at all These substances vary in the different species almost to infinity, taking into their constitution, in addition to the four organogens, minute portions of the mineral substances introduced by rain and river water. Their peculiarities of odor, flavor, color, properties, etc., although so obvious to the senses, are occasioned by differences of constitution often so slight as to elude the most delicate tests of the chemist.

857. The following table contains examples of the various classes of secretions, arranged in reference to their relative proportion of oxygen:-