The potential for interspecific hybridization within the genus Eucalyptus was investigated through controlled pollination and measurement of seedling leaf morphology. Eucalyptus gillii and E. socialis (subgen. Symphyomyrtus sect. Bisectae ser. Subulatae) were used as the female parents, and pollen was sourced from 16 Eucalyptus species from a number of series within sections Bisectae and Adnataria (subgen. Symphyomyrtus). Thirty-four out of 36 crosses produced seeds; however, the percentage of seeds produced per flower pollinated varied considerably between crosses, as did germination percentage and seedling survival. At 3 mo of age, all surviving seedlings were measured for 15 leaf and stem characters. Multivariate analysis (ordination) of the data from each cross placed the seedlings in relation to their parents, with most crosses intermediate, albeit closer to the maternal parent. Successful hybridization occurred within sect. Bisectae ser. Subulatae and between ser. Subulatae and sect. Bisectae ser. Kruseanae, ser. Levispermae, ser. Curviptera, ser. Erectae and sect. Adnataria ser. Heterophloiae. In contrast, crosses between sect. Bisectae ser. Subulatae and sect. Adnataria ser. Aquilonares and ser. Melliodorae did not produce hybrids. Crosses between closely related species showed a greater degree of success than those between distant crosses, as did those between species with similar flower size.

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