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xvii been exposed through the medium of the leaves to the action of the atmosphere.

These elaborated matters and the cellular substance itself are all formed of oxygen, hydrogen, carbon, and nitrogen, combined in various proportions. Carbon, the most abundant element in the solid parts of all vegetables, is derived chiefly, if not entirely, from the small quantity of carbonic acid existing in the air; nitrogen, which only enters into the composition of a few vegetable products, is obtained from the nitrates and ammoniacal salts present in the soil; while the two other elements are probably due to the decomposition of a portion of the water taken up by the root, the superfluous oxygen being exhaled by the leaves during the day. Thus plants form a necessary part of the great cycle of Nature, absorbing those gaseous particles produced by the respiration of animals and the decay of organic matter, and restoring to the air, in a purified state, that principle which is necessary to the support of all animate being.

Water forms a considerable portion of growing vegetables, and is necessary to their existence. It is chiefly drawn up by the fibres of the root, though some plants doubtless absorb much from the atmosphere. Being required in very varying proportion by different species, the amount present in the soil has a great influence upon its vegetation. Some plants will only grow entirely submersed, others flourish in the stagnant bog or peat-moss, some require the moist but well-drained hill-side, and a few are found in the most arid localities, where occasional dews form the only source of moisture.

In addition to these elementary components, vegetables always contain a considerable quantity of various other substances of inorganic origin. Potash and soda are the most prevalent of these; they are present in all plants, the former alkali existing in by far the greater proportion in most inland species, while an abundance of the latter is characteristic of those growing near the sea, being