KONG, HONGZHI*, ANMING LU, and ZHIDUAN CHEN. Laboratory of Systematic and Evolutionary Botany, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, China. - Floral organogenesis of Chloranthus sessilifolius K. F. Wu (Chloranthaceae) with special emphasis on the morphological nature of the androecium of Chloranthus Sw.
Whether the androecial structure of Chloranthus represents a
single stamen with four pairs of sporangia or three independent
stamens that have undergone fusion with one another towards their
base, has remained a major controversial issue. In literature, it has
been described as "a single stamen", "three
stamens", "three anthers", "a single tripartite
anther", or "a three-lobed filament". Two classes of
hypotheses were also proposed to interpret its evolutionary process,
but no accordance has reached yet. In this study, we observed the
floral organogenesis of C. sessilifolius K. F. Wu, a perennial
herb with androecial lobes cohered only at the base. Inflorescence
primordium, produced at the end of leafy shoots, is dome-like at the
beginning and then elongates upwards, from which bract primordia
initiate almost decussately. Floral primordium, arising from the axil
of the bract, soon becomes a scale-like structure, with androecial
primordium originating from its adaxial margin, and gynoecial
primordium in its abaxial position. As three primordia of androecial
lobes become visible in the adaxial margin of the floral primordium,
four thecae are already in differentiation, and the gynoecial
primordium appears as a shallow disc. The androecial lobes do not
extend its length until the thecae approach maturity, and the
elongation of the androecial lobes is simultaneous with the
differentiation of the stigma. Ontogenetically, C.
sessilifolius shares many character states with Sarcandra
species, and more with C. spicatus (Thunb.) Makino. Based on
these results and those of Endress, combined with evidence from
neobotany, palaeobotany and molecular systematics, the morphological
nature of the androecium of Chloranthus is further discussed. The
second hypothesis of Endress, that the androecial structure of
Chloranthus might have arisen by splitting of a single stamen with 2
marginal thecae, seems to be the most plausible interpretation.
Key words: androecium, Chloranthaceae, Chloranthus, floral organogenesis