Floral morphology and the development of the gametophytes in Eucalyptus stellulata Sieb

Abstract

Bud formation on new growth is a continuous process which extends from early spring until midsummer, and at any time except during the winter months a wide range of floral development is exhibited on each tree. The main flowering period in the Armidale district is late summer to early autumn but sporadic flowering may occur at any time.

The development of the single operculum and the floral parts is traced but, owing to the prolonged period over which bud primordia are formed, stages of organogenesis cannot be related to definite seasons. At the onset of meiosis the buds enlarge and rupture the two fused bracts which enclose the inflorescence, and these are shed.

Embryo sac formation is of the Polygonum type and the components of the egg apparatus undergo a threefold increase in size before anthesis. In more than half the embryo sacs the endosperm mother cell becomes multinucleate and its subsequent degeneration is followed by that of the egg apparatus. Such abnormal ovules continue their growth and cannot be distinguished externally from those containing normal embryo sacs.

The fertile pollen grains are two-celled when shed but a high proportion of flowers produces only sterile pollen whose development has been arrested at the one-nucleate stage.