Several varieties of this well-known tree are cultivated, differing in the size of the flowers, in the form of the foliage, <fcc., such as the “oak-leafed” (quercifolvum), pendulum, crispum, &c. One of the most remarkable forms is C. Adami, Poir. (C. purpurascens, Hort.), which bears three kinds of blossoms, viz., racemes of pure yellow flowers, others of a purple colour, and others of an intermediate brick-red tint—all three kinds being borne by one and the same tree. The last are hybrid blossoms, and are sterile, with malformed ovules, though, curiously enough, the pollen appears to be good. The yellow and purple “reversions” are fertile. It originated in Paris in 1828 by Μ. Adam, who inserted a “shield” of the bark of C. purpureus, Scop., into a stock of C. Laburnum, L. A vigorous shoot from this bud was subsequently propagated. Hence it would appear that the two distinct species mentioned above became united by their cambium layers, and the trees propagated therefrom subsequently reverted to their respective parentages in bearing both yellow and purple flowers, but produce as well blossoms of an intermediate or hybrid character. Such a result, Mr Darwin observes, may be called a “graft-hybrid.” For full details see Darwin’s Animals and Plants under Domestication.

The laburnum has highly poisonous properties. A case is recorded of nearly fatal results to several boys who masticated the roots on finding they tasted like liquorice, which is a member of the same family as the laburnum. It has proved fatal to cattle, though hares and rabbits eat the bark of it with avidity (Gardener's Chronicle, 1881, vol. xvi. p. 666). The seeds also are highly poisonous, possessing emetic as well as narcotico-acrid principles, especially in a green state. Gerard (loc. cit.) alludes to the powerful effect produced on the system by taking the bruised leaves medicinally. Pliny records that bees will not visit the flowers (W. H., xvi. 31), but this may be an error, for Mr Darwin found by experiment that insects play an important part in the fertilization of the laburnum.

The heart wood of the laburnum is of a dark reddish-brown colour, hard and durable, and takes, a good polish. Hence it is much prized by turners, and used with other coloured woods for inlaying purposes. The laburnum has been called false ebony from this character of its wood.

The roots are subject to a peculiar disease, not at all uncommon in other members of the Leguminosae, the fine rootlets swelling into minute club-shaped processes called exostoses, resembling coral-branches in shape. Large masses of such, one or two inches in diameter, may be found at the extremities of the roots of old laburnum trees. They are apparently caused by a fungus which appears to be ubiquitous, as the disease is rarely, if ever, known to be absent, though it does not seem to cause much if any injury to the health of the plants it attacks. See Studier ofver Leguminosernas rotknolär, 1874, by Dr Jacob Erikssen: also Gardener's Chronicle, 1879, xi. p. 209, and xii. p. 112.