Magazine pp. 289-290 (June 1840)
ART. II. On the singular Origin of the Purple Laburnum, and on the new Field
which it opens to the Horticulturist for the Production of Hybrid Plants.
By the Hon. and Rev. W. HERBERT, D.C.L. F.H.S. &c.
I AM much obliged to you for the opportunity you have given me of reading M. Poiteau's interesting account of the origin of the purple laburnum, or Cytisus Adàmi, which I had understood to be an accidental hybrid produced in the garden of M. Adam, but which is stated, and (I doubt not) correctly, to have originated from a graft of C. purpùreus, of which the bud had perished. I am not aware what the hypothesis of M. Prevost and M. Leclerc, to which M. Poiteau alludes, may be; but I think I understand how this singular plant must have been produced, and, if I am right in my notion, it opens a field for the horticulturist to produce hybrid plants which perhaps could not be obtained by seed. It is asserted, that, long after the bud on the graft had perished, other small buds formed themselves round it, all of which produced the true C. purpùreus, except one, from which proceeded the extraordinary hybrid. I apprehend, that, if attention had been paid to this phenomenon, it would have been found that the bud which produced it was formed exactly at the junction of the bark of the two species, and that the two contributed equally to its formation. A similar effect might perhaps be expected also from a bud formed where the mere bark of the graft is in contact with the wood of the stock. Every bud on a tree is an individual; and, if the graft and the stock from any peculiar circumstances contribute equally to the formation of a new bud, the individuality of that bud may be expected to partake of their joint natures, as much as that of the plant which is raised from hybrid seed. Let it therefore be the object of gardeners who wish to obtain new plants analogous to the C. Adàmi, to kill the bud of the graft after a perfect union has taken place, and to try to force the plant to break again from the seam or edge of the bark that has been inserted. Unless the bud shall be formed on the very seam, or where the bark inserted is thin, so that the bud shall have taken its rise from the contributive powers of the two plants, a new formation cannot be expected.
The circumstances attending the growth of C. AdÓmi are very singular. Upon your tree one branch had reverted nearly to the type of the laburnum, and another nearly to that of C. purp¨reus, while the central shoots retained the hybrid character; but, on close observation, neither the leaves nor the flowers of the two branches, which had so reverted to the elements of the parents, were precisely similar to them; both however had acquired fertility, while the central shoots continued sterile. From the seed produced on the yellow-flowering branch, several plants have been raised, which have the general aspect of the laburnum: but in my brother's garden one of them has grown to the height of 4 ft. 6 in., while another beside it remains only 2 in. high; and, amongst those I raised, two showed a purple stain on the young wood and petioles. The seed produced on your small-leaved branch did not vegetate, but I have a seedling two years old from such a branch on my brother's tree, which has entirely the aspect of C. purp¨reus, though differing a little in the shape of the lobes of the leaves. The branches of the small-leaved variation upon his were last year loaded as with a sheet of small purple flowers, but a branch destined to bear yellow flowers having made its appearance, upon its first producing blossom this season, not a flower appears upon the small-leaved portion. Another strange circumstance has occurred, as he informs me that a strong rigid branch last year shot from the tree perpendicularly downwards, of which we must await the further developement.—London, May 7. 1840.