Operculum development of 62 species in seven subgenera of Eucalyptus was traced by rapid freeze-sectioning techniques and a comparative study was made of the perianth in Angophora. Four principal pathways of operculum development were detected. In those species having two perianth whorls, sepal and petal primordia were always initiated sequentially but with variation in the time involved between their appearance. In those with a single operculum shown to be calycine, the petaline whorl was totally suppressed.
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Pryor et al. (1971) studied this question.