TREE TREFOIL; a genus of the deciduous order, belonging to the diadelphia class of plants. There are 11 species, of which the most remarkable are:

1. The laburnum; or large deciduous cytisus, hath a large upright tree-fern, branching into a full-spreading head, 20 or 30 feet high, having smooth greenish branches, oblong oval entire leaves, growing by threes on long slender foot-stalks; and from the sides of all the branches, numerous yellow flowers collected into long spikes, hanging loosely downward, and appearing in May.

2. The fellifolius, often called cytisus secundus clusii, have a low shrubby stem dividing into numerous erect brownish branches, forming a bushy head five or six feet high, garnished with small oval leaves growing by threes; some on very short footstalks, others fitting close; and bright yellow flowers in short erect spikes at the ends of the branches, appearing in June.

3. The nigricans grows with a short shrubby stem, dividing low into many erect slender branches, forming a bushy head four or five feet high, with oblong, oval, trifoliate leaves, and yellow flowers, terminating all the branches in upright spikes, appearing in July.

4. The hirtiflora, or hairy evergreen Neapolitan cytisus, rises with an upright shrubby grey stem, sending out many erect greenish hairy branches, forming a fine head six or eight feet high, closely garnished with small hairy trifoliate leaves on short footstalks, and yellow flowers from the sides of the branches in short pendulous spikes, appearing in June.

5. The Aufricus, Austrian, or Tartarian evergreen cytisus, hath a shrubby stem, dividing low into many greenish branches, forming a bushy head three or four feet high, having smooth whitish-green leaves, and bright yellow flowers in close umbellate heads at the ends of the branches, having a cluster of leaves under each head. These flowers appear in May.

Culture, &c. All the sorts are hardy, and will prosper in any common soil and exposure; though, as the hirtiflora is sometimes affected by severe frosts, it should have a dry soil, and a somewhat sheltered situation. They may all be propagated by seeds or cuttings, and all the culture they require in the nursery is to have the ground kept clear from weeds, and dug annually between the rows. Though they are generally considered only as ornamental shrubs, yet the first species, if originally trained to a stem, and suffered to stand, will grow to the size of pretty large timber trees. They grow naturally on the Alps, the mountains of Dauphine, and the highlands of Scotland; and the timber being very hard, and taking a fine polish, is frequently used for making chairs, tables, bedsteads, and other furniture; and is said to equal the finest mahogany in beauty. A species of cytisus, called by Linnaeus cytisus cajan, is known in the West Indies, where it is a native, by the name of the pigeon-pea, from the seeds being the common food of these birds in that part of the world. These seeds are also sometimes used as food for the human species; and as they are of a very binding quality, afford a wholesome nourishment during the wet season, when dysenteries are so frequent.