The species of the genus citrus are not very clearly defined, and some botanists have regarded the citron, orange, shaddock, lime, and lemon, usually considered distinct, as forms derived from one species, the citron (C. medico); but whatever may be the difficulties presented to the botanist, horticulturally and commercially they are sufficiently distinct in the form and sensible properties of their fruit. The lemon grows wild in the north of India, and has long been in cultivation among the Arabs, who introduced it in various parts of Asia and Africa; its introduction into Europe is accredited to the crusaders, but the precise date is unknown; it is now naturalized in the West Indies and other parts of America. The botanical characters of the genus will be given under Orange, from which the lemon differs in but few particulars except the fruit; the petiole of the orange leaf has a broad wing upon each side, while in the lemon this is very small or wanting; the flowers, usually smaller than those of the orange, have the exterior tinged with purple; the usually elongated fruit has a projection or nipple at the end opposite the stem.

The oil, with which the rind abounds, has a different odor from that of the orange, and the juice, rich in citric acid, is intensely sour; to this, however, the variety known as the sweet lemon is an exception. The lemon succeeds in the same climates as the orange, and the culture of the two is the same. Over 30 varieties are enumerated, some of which, as the horned and the fingered lemons, are only known in the collections of amateurs as curious monstrosities. Those in cultivation for profit differ in size, shape, thickness, and roughness of skin, and the size and form of the nipple at the end. In a horticultural classification they are grouped as round, pear-shaped, cylindrical, gourd-shaped, wax, etc, with several varieties under each head; in commerce they are known by the names of the ports from which they are shipped, as Messina, Lisbon, etc. Very fine lemons are produced in Florida and in the southern part of California, but much less attention has been given to their cultivation than to that of the orange; as the orange is more difficult to transport than the lemon, it always brings a higher price, and the growers find it more profitable.

The lemon tree is frequently cultivated in conservatories, and is a favorite as a house plant, as it is ornamental in its foliage, flower, and fruit; if not exposed to too low a temperature, the tree will survive a great deal of bad treatment, and those seen in house culture are usually in a poor condition. The tree should have a good open soil, and when not in a growing state during the winter needs but little water; new growth begins late in winter or early in spring, when it should be watered freely, and when the young wood has hardened the plant set out of doors where it will be sheltered from violent winds. The fruit, which sets in spring, remains upon the tree all winter, gradually coloring. The foliage must be washed occasionally to remove dust, and a smutty fungus which sometimes appears; and if a scale insect is found, it must be removed by the use of strong solution of soap applied with a stiff brush. The lemon is valued for its acid juice and its aromatic rind, and its domestic uses in making cooling drinks and for flavoring are well known.

The juice contains nearly two Per cent. of citric acid with mucilage and bitter extractive matter. (See Citric Acid.) - The oil of lemons is contained in receptacles in the outer portion of the rind; it is a volatile oil, and was formerly obtained by distillation, but the oil prepared in this manner has a less pleasant flavor than that by expression. The outer portion of the rind, or flavedo as it is called, is removed by rasping and afterward subjected to pressure; after resting to deposit its coarse impurities, it is filtered and put into copper cans of about six gallons capacity, in which it is exported; the supply comes from the south of Europe. This oil has the same composition (C10H8) as the oil of turpentine, and when kept for a long while it loses its proper flavor and has that of turpentine. The oil is largely used by confectioners and pastry cooks for flavoring; the extract of lemon sold for domestic use is a more or less concentrated solution of the oil in alcohol; when mixed with alcohol the oil retains its purity of flavor much longer than when kept by itself, and perfumers preserve this and other essential oils from deterioration by mixing them with alcohol as soon as received.