Comparative floral development and androecium structure in Malvoideae (Malvaceae s.l.)

Floral development and androecium structure were studied in eight genera of the Malvoideae (Malvaceae s.l.). All core Malvoideae (Malvaceae s.s.) share a uniform pattern of floral development and have several characteristic androecium structures in common. A corolla/androecium ring wall is formed and subsequently differentiated into five androecial sectors (= primary androecial primordia) that give rise to the androecium and corolla. Petals are developed at the base of the androecial sectors and are shifted slightly clockwise or counterclockwise relative to the androecial sector. Secondary androecial primordia are initiated centrifugally in two rows on each sector in an alternate pattern. The first secondary androecial primordium is initiated on the opposite side of the androecial sector from the petal. The secondary androecial primordia later subdivide longitudinally and differentiate into functional androecial units, which each have a two-locular theca with a single longitudinal dehiscence zone. In some taxa, sterile tips are formed at the distal end of the androecial tube in an antepetalous position. In Matisieae, sister to core Malvoideae, the early floral development is nearly identical to that in core Malvoideae. In later development and at anthesis, Matisieae and core Malvoideae show more pronounced differences. In Matisieae, a subdivision of secondary androecial primordia occurs generally transversely as compared with longitudinally in core Malvoideae. While the androecium of core Malvoideae consists of a long tube with lateral stalked androecial units, the androecial tube of Matisieae is stouter and bears sessile androecial units mostly on lobes. These lobes, however, do not correspond to the androecial sectors, as do the sterile androecial tips of core Malvoideae. The sterile androecial tips of core Malvoideae have the same position as androecial units at the tip of androecial sectors in other Malvaceae and could be remnants of these structures. The androecial lobes in Matisieae are, however, positioned alternipetalously and might correspond to remnants of a second whorl of androecial structures found in other Malvaceae or Malvales.