Prunus is a most variable genus. This is indicated by the several sub-genera, the large number of species and the various arrangements of these groups by different authors. At their extremes sub-genera and species are very distinct, but outside of the normal types, and sometimes in several directions, there are often outstanding forms which establish well-graded connections with neighboring groups. For example, among the American plums there are but few species between which and some other there are not intermediate forms that make the two species difficult to distinguish under some conditions. There is also a wide range of variation within the species. The modifications within the species are oftentimes such as to change greatly the aspect of the plant; the trees may be dwarf or luxuriant, smooth or pubescent; may differ in branching habit, in leaf-form, in size and color of the flowers, in the time of opening of leaf and flower-buds, in color, shape, size, flesh, flavor and time of ripening of fruit, in the stone and in all such characters as climate and soil environment would be liable to modify. This inherent variability is one of the strong assets of the genus as a cultivated group of plants, for it allows not only a great number of kinds of fruits and of species but a great number of varieties. Besides, it gives to the genus great adaptiveness to cultural environment, in accordance with climate, location, soil and the handling of the trees. The cultivator is able to modify, too, the characters of members of the genus to a high degree in the production of new forms, but few, if any, groups of plants having produced as many cultivated varieties as Prunus. The genus Prunus is preeminent in horticulture, furnishing all of the so-called stone-fruits, fruits which for variety, delicious flavor and beauty of appearance, probably surpass those of any other genus, and which, fresh or dried, are most valuable human foods. The seeds of one of the fruits belonging to Prunus, the almond, are commercially important, both for direct consumption and for the oil which is pressed from them; in India a similar oil is obtained from the seeds of peaches and apricots, while in Europe an oil from the seeds of the Mahaleb cherry is used in making perfumes. Various cordials are made from the fruits of the several species, as kirschwasser and maraschino from cherries, zwetschenwasser and raki from plums, and peach brandy from the peach; while fruits and seeds of the several species are soaked in spirits for food, drink and medicinal purposes. The bitter astringent bark and leaves are more or less used in medicine as is also the gum secreted from the trunks of nearly all the species and which, known as cerisin, is used in various trades. The wood of all of the arborescent species is more or less valuable for lumber, for cabinet-making and other domestic purposes.