Floral ontogeny in legume genera Petalostylis, Labichea, and Dialium (Caesalpinioideae: Cassieae), a series in floral reduction

Floral ontogeny of taxa of two subtribes (Labicheinae, Dialiinae) of caesalpinioid tribe Cassieae, characterized by reduced number of floral organs, was compared. All three taxa studied are distichous; Petalostylis labicheoides flowers are solitary in leaf axils, Labichea lanceolata has few-flowered racemes, and Dialium guineense has numerous-flowered cymes. The first sepal primordium in each is initiated abaxially and nonmedianly. Order of organogenesis in Petalostylis is: five sepals bidirectionally, five petals and carpel simultaneously, then five stamens bidirectionally, starting abaxially. The order in Labichea is: five sepals helically (one lagging in time), five petals unidirectionally starting abaxially, the carpel and petals concurrently, then two stamens successively, starting laterally. Order in Dialium is: five sepals bidirectionally, the single petal adaxially, and lastly the carpel and two stamens concurrently. Specializations include (1) reduction of the five sepals to four by fusion in Petalostylis and Labichea; (2) reduction of petal number to one in Dialium; (3) reduction of stamen number to two in Labichea and Dialium, and reduction of functional stamens to three in Petalostylis; and (4) an elaborate, late-developing style in Petalostylis. Floral asymmetry, another specialization, characterizes Labichea, expressed by dissimilar stamens, while the other genera have zygomorphic flowers. Floral ontogenies are compared with other taxa of Cassieae. The stimulus for the present work was the conclusion, in a cladistic analysis of caesalpinioid taxa based on floral developmental and morphological characters (Tucker and Douglas, 1994), that caesalpinioid tribe Cassieae does not appear to be monophyletic. Two distinct components include: a monophyletic subtribe Cassiinae (Cassia sensu stricto, Chamaecrista, and Senna; studied in Tucker, 1996a), and Ceratonia and Dialium, forming a grade near the base of the legumes. Consequently, I wanted to explore and compare other taxa of Cassieae in more detail, to determine trends of specialization in the tribe and to identify divergent pathways of floral development. Floral development in Ceratonia (Tucker, 1992), the only genus of subtribe Ceratoniinae, and in representative taxa of the Cassiinae (Tucker, 1996a) had been studied. In a recent symposium volume (Tucker, 1996b), the poricidal stamens common in Cassieae were described. In the present paper, floral development of the Australian genera Petalostylis and Labichea, comprising subtribe Labicheinae of caesalpinioid tribe Cassieae, and of African and South American species of Dialium in subtribe Dialiinae, are compared. This work permits struc