Phyllotaxis and morphology of proximal leaves on vegetative axillary shoots of Salix and their systematic implications

摘要

This investigation concerns the proximal leaves on axillary shoots. Proximal leaves are defined as the first four leaf-like organs (1) to (4) of the middle shoot (not counting stipules and the organs of the lateral buds) inside the outer tubular bud scale. These may be scale-like or incompletely developed foliage leaves. The morphology of these proximal leaves, their angular positions with respect to the mother shoot, and the sense of rotation of the phyllotactic spiral were recorded in plants from 59 Salix taxa (species, subspecies, hybrids). The mean position (a_1 of the first proximal leaf = leaf (1) is defined as the divergence angle between this leaf and the median plane of the axillary bud. It ranges from -7° to +91°. The phyllotaxis of the proximal leaves of the various Salix species corroborates to some degree the intrageneric classification by Skvortsov (1968). In some species with the first proximal leaf close to 0°, this leaf had characteristics of a cataphyll, i.e. an additional bud scale. This is interpreted as a relictual characteristic from common ancestry with Populus. In species with divergence angles a_1 between +35° and +75°, homodromy (i.e. the same sense of rotation in succeeding shoot generations) occurred with a significantly higher frequency than the expected 50%.