Phyllotaxis in Eucalyptus socialis F. Muell and E. oleosa F.Muell

Abstract

The occurrence of unusual phyllotaxis in seedlings of E. oleosa led to a comparative study being made between these and seedlings of E. socialis. The seedling leaves of E. socialis were found to remain opposite and decussate for approximately 10 pairs, after which the basic decussate arrangement underwent a gradual modification to produce the apparent alternate phyllotaxis that characterizes mature trees of most Eucalyptus species. In contrast, the seedlings of E. oleosa produced two to seven pairs of opposite and decussate leaves. The disposition of the next few leaves usually fitted no pattern that could be readily determined, but there was an organization of the subsequent leaves at successive pairs of nodes into a spiral. A few specimens reverted to a decussate arrangement during the period of study. The final arrangement of the mature leaves of both forms was "alternate", but the difference in the arrangement of the seedling leaves provided a useful marker for the separation of the two taxa. Seedlings which exhibited triphylly and tetraphylly were also examined. Anatomical examinations of seedling apices were made to observe the order of formation of leaf primordia of both spiral and decussate forms.