Developmental morphology of the floral organs of Eucalyptus. I. The inflorescence

Abstract

The nature and structure of the inflorescenoe of Eucalyptus is re-defined in the light of an analysis of the unit inflorescenoe, its development, mode of branching, bract systems, and the grouping of unit inflorescences into compound inflorescences.

The structure of the unit inflorescence in 80 representative species is elucidated and a systematic notation for expressing the numbers and arrangement of the involucral bracts in dichasial inflorescences is proposed.

The infloresoence bud of Eucalyptus is generally asymmetrical in side view and often also in face view. External asymmetry is largely a result of the asymmetry of the outermost pair of bracts, by which the developing inner bracts and flowers are at first tightly enclosed, and may be reflected in an asymmetrical pattern of branching within the inflorescence. The asymmetry and pattern of branching are reflections of morphological features peculiar to the shoot system of Eucalyptus. The outer bracts resemble the adult leaves in their asymmetry about the midrib. The leaf and bract primordia arise decussately at the shoot apex but in most species the leaves of each pair become separated by the development of a segment of axis (an "intranode") between them.

In certain species the development of an intranode between the outer pair of bracts leads to the splitting apart or "disarticulation" of the unit inflorescence into two subclusters. The very complex inflorescence of E. camaldulensis arises by disarticulation of a simple inflorescence, followed by interpolation, in a centripetal manner, of additional bracts and flowers between those first formed. Flower numbers in clusters at successive nodes of the annual shoot may vary, rising with node number if the inflorescences are basitonically disposed, falling if acrotonically disposed. Flower number in successive clusters is correlated with other quantitative features of the annual shoot.

The position and number of the inflorescenoe buds on the annual shoot and the interval between flower initiation and anthesis are characteristic of species and of groups of species. Phylogenetic trends in inflorescence characters are discussed. Inflorescences with few flowers and with free persistent bracts, the number of which is related to the flower number in the cluster, are held to be primitive. Inflorescences with many flowers, fused bracts constituting a caducous involucre in which the number of bracts is not related to the number of flowers, are held to be advanced. These and other charaoters of the infloresoence are shown to have taxonomic significance and their use is typified in an analysis of the infloresoence characters of the Renantheroideae in relation to those of the Renantherae.