Note 518 — Chapter III · ENATION.

It is desirable in this place to allude to a singular case of fissiparous division of a leaf of Prunus Laurocerasus described by Prof. Alexander Dickson ('Seemann's Journ. Botany,' vol. v, 1867, p. 323), and which did not come under the writer's notice till after the sheet relating to fission, p. 61, had been sent to press. Dr. Dickson thus speaks of this abnormal leaf:—"The petiole (unchanged) supported two laminæ, placed back to back, and united by their midribs (i.e. not separated) to within about an inch from their extremities, which were perfectly free from each other. These laminæ stood vertically, their edges being directed towards and away from the axis; and as they were placed back to back, the shining surfaces, corresponding in structure to the normal upper leaf-surface, were directed laterally outwards. In the axil of this abnormal leaf were two axillary buds. The existence of two leaf-apices and two axillary buds shows that this was not due to an accidental exuberance of development, but to fissiparous division, which, had it been complete, would have resulted in the replacement of a single leaf by two leaves. The arrangement in Prof. Dickson's leaf may be thus represented: [symbol:)OO(with X above]. The nature of the case may be even better seen by comparison with the normal arrangement, which would be [symbol: (OX turned 90 degrees ccw], while in those cases where the fission of the leaf occurs in the same plane as that of the primary lamina, as where a leaf splits into two lobes at the apex, with a midrib to each, the arrangement is as follows: [symbol: OX turned 90 degrees ccw, with 2 arcs below], the X in all cases representing the position of the axis, the O that of the axillary bud, and the [symbol: (turned 90 degrees ccw] that of the laminæ."