This order has been divided into four sections; three of which contain well-known plants, and have been divided into three orders by many botanists. The Rue (_Ruta graveolens_) is well known from its strong and disagreeable smell, which is produced by the oil secreted in transparent cells in the leaves, which have the appearance of dots, when the leaves are held up to the light. The leaves are of a bluish green, and the flowers of a greenish yellow; the latter growing in cymes at the end of the branches. There are four sepals, four petals, and eight stamens. There are four carpels, seated on an elevated receptacle, and each containing one cell, which grow into a four-celled fruit. In Fraxinella (_Dictamnus_) the petals are unequal; there are ten stamens, one style, and the carpels are two-seeded. In Diosma there are only five stamens, the style is arched, and the capsule consists of five-horned carpels. In Corræa the leaves are opposite; there are eight stamens, and the four petals grow together into a tube at the base; and in Crowea there are five sepals, five petals, and ten stamens; the leaves are also alternate. The Diosmas have as strong a scent as the Rue, and a perfume is made from them called Bucku at the Cape of Good Hope, of which country they are natives.
The section Zanthoxyleæ contains the Zanthoxylum, also called the Toothache Tree, or Prickly Ash, a native of North America, the bark of which is very fragrant, and is said to be a cure for toothache and rheumatism; Ptelea or Shrubby Trefoil; and _Ailantus glandulosa_. _Zanthoxylum fraxineum_ has very pretty pinnate leaves, and small purple flowers; _Ptelea trifoliata_ has curiously winged fruit, which resemble those of the elm; and the Ailantus has remarkably long compound leaves, one leaf having been known to have fourteen pairs of leaflets, and to be upwards of three feet long. The two following orders are included in Rutaceæ by some botanists.
_Quassia amara_, the bark of which is sometimes used as a substitute for hops, is perhaps the best known plant belonging to this order. All the species are trees or shrubs, natives of tropical America, with bitter bark, milky juice, and pinnated leaves.
Tropical shrubs with yellow flowers and shining leaves; seldom seen in British hothouses.
