31 THE ANATOMY AND MORPHOLOGY OP THE OPERCULUM IN THE GENUS EUCALYPTUS. RT I. THE OCCURRENCE OF PETALS IN EUCALYPTUS GUMMIFERA (GAERTN.) HOCHR. By J. L. Willis, Museum of Applied Arts and Sciences, Sydney. (Plates ii-iii, and two Text-figures.) [Read 26th April, 1951.] Synopsis. The nature of the operculum present in the genus Eucalyptus has been for many years the subject of a considerable amount of conjecture, and a number of conflicting interpretations as to its morphological nature have been proposed. In the young buds of E. gummifera, four minute imbricate petals have been found, which gradually fuse together to form an inner coroUine operculum. This inner operculum remains quite distinct from the outer operculum which Is probably calycine in origin. There is, therefore, a close morphological relationship between the flowers of this species and the two New Caledonian genera Piliocalyx and Acicalyptus, which may also indicate a close phyletic relationship. Introduction. Although a considerable volume of literature dealing with the anatomy of the genus Eucalyptus has accumulated, no investigations have yet been carried out on that unique organ, the operculum, most attention having been focussed upon the wood anatomy, leaf structure, etc. This is somewhat surprising, as the exact nature of the operculum has been the subject of a considerable amount of conjecture since 1788 when the genus was first described by L'Heritier. Historical. In his original description, and after consideration of the one species only (E. obliqua), L'Heritier held the operculum to be corolline in nature. On the other hand, Jussieu (1812) considered that it was formed by the fusion of two bracts. Robert Brown (1814), however, came to the conclusion that the operculum had different origins in different groups of species. He thought that in most species it represented a fusion of the calyx and corolla; in those species with double opercula, the inner structure represented the corolla, and the outer one the calyx; and in the genus Eudesmia R.Br., now Eucalyptus L'Her., Series Eudesmiae (Blakely, 1934), the operculum was formed from the corolla alone. Hooker (1860) concluded that the operculum was a combined calyx and corolla, but Bentham (1866) was uncertain as to the correct interpretation of the organ, although he considered it to be most likely corolline in nature. He noted the presence of an additional outer operculum in some species, but regarded the nature of this outer organ as doubtful. Bentham thought that this outer operculum would eventually be found to be present in nearly all species but that it was deciduous so early that it was not noticeable in most buds. Maiden (1923) regarded the operculum as corolline in origin except in those species with double opercula where he considered the inner one to be corolline and the outer one calycine. He predicted that eventually all species would be found to have double opercula, the outer calycine one being deciduous very early in most species. Naudin (1883), Deane (1900), Andrews (1913), Hutchinson (1926), Blakely (1934), Rendle (1938), and Osborne (1947) all considered the operculum to be formed from the fused petals, whereas von Mueller (1879-84) concluded that the organ was nearly always calycine in nature. Hardy (1935, 1939), however, thought the operculum was either a modified coi'olla or a fusion of both calyx and corolla, but he considered the evidence insufficient to determine definitely which interpretation was the correct one.