689 
Floral development in the Metrosideros 
group (Myrtaceae) with special emphasis 
on the androecium 
D.A. Orlovich, A.N. Drinnan and P.Y. Ladiges 
Abstract 
Orlovich, D.A., Drinnan, A.N. and Uidiges, P.Y. (School of Botany, The University of Melbourne, ParkviUe, 
Victoria 3052, Australia) 1996. Floral development in the Metrosideros group (Myrtaceae) with special 
etnphasis on the androecium. Telopea 6(4): 689-719. The floral development of eleven taxa in the 
informal 'Metrosideros group' (Myrtaceae) is described. Two general patterns of androecial 
development are identified. The multistaminate fascicles of hophostemon confertus, L. suaveoiens, 
L. lactifluus, Welchiodendron longivalve and Tristaniopsis iaiirina develop from stamen primordia 
initiated on antcpetalous pre-staminal bulges. In Xanthostemon oppositifolius, X. verticillatus and 
Lysicarpus angnstifolius, which have a complete ring of unfused stamens, often with more stamens 
in the antcpetalous region, pre-staminal bulges do not develop prior to stamen initiation; stamen 
primordia are initiated directly on the floral tube. Differential hypanthial expansion results in the 
spreading-out of antesepalous stamens whilst antepetalous stamens remain clustered in front of 
the petals. Metrosideros collimt var. viiiosa, Tristania neriifolia and Thaleropia queenslandica have fewer 
stamens, which develop on either small pre-staminal bulges (M. coiiina) or on the flank of the 
invaginated floral apex. Staminal arrangement in all the taxa studied is a result of the interaction 
between the timing of androecial initiation and the available space on the hypanthium. 
Introduction 
The family Myrtaceae is large, diverse and widely distributed in the southern henaisphere, 
where it is a prominent component of the Australian flora, including 75 native genera, of 
which 55 are endemic. Based on morphological characters, Johnson and Briggs (1984) 
presented a phylogenetic analysis of Myrtaceae as part of a broader study of the order 
Myrtales. Roral characters included those of the androecium, which shows a variety of 
form in mature flowers. Within Myrtaceae, stamens vary in number, colour, length, position, 
whether they form clusters or not, and whether stamen filaments are united or free. 
Schmid (1980) discussed staminal placement in Myrtaceae in terms of diplostemony, 
haplostemony, obdiplostemony, and obhaplostemony. Johnson and Briggs (1984) argued 
for a more interpretative, rather than wholly descriptive approach, and presented an 
hypothetical scenario of sequences of androecial evolution. They suggested that the 
'origin of the family more or less coincides with the proliferation of stamen initials 
within each of the ten primordia corresponding to an earlier ancestral 5-merous, 
2 whorled condition' (p. 739). They concluded, thus, that the original androecial 
condition in Myrtales was diplostemonous (10 subequal stamens), but that, in 
Myrtaceae, this gave way first to obdiplostemony (10 stamens with smaller stamens 
opposite sepals) and then to 10 stamen fascicles, the antesepalous fascicles being small. 
From this they derived the range of conditions observed in mature flowers, including 
only antepetalous (obhaplostemonous) stamens or fascicles, rings of stamens, or 
reduction or suppression of stameris (Johnson and Briggs 1984, Fig. 8, p. 740). 
Although variation in form of mature myrtaceous flowers is relatively well known, 
developmental studies are needed for character definition (assessment of homology) 
and as evidence for character polarity (transformation from plesiomorphic to 
