Studies in the Alismataceae. X. Floral organogenesis in Luronium natans (L.) Raf.

Abstract

Floral organogenesis of Luronium natans (L.) Raf. occurs at first in an alternating trimerous pattern typical of Alismataceae, with the formation of three sepals, then three bulges, corresponding to the petal-stamen primordia described in some other Alismataceae, alternating with the sepals. A petal is initiated on each bulge and a pair of stamens is initiated either on it or close to it. After this, development no longer follows a trimerous plan. Six carpels are initiated in positions alternating with the six stamens, and further carpels may then arise above and between the first six. The carpels ultimately lie in a whorled arrangement if there are only six; if more, they may appear whorled or irregularly arranged. After the initiation of the stamen pairs, floral organ primordia appear simply to be positioned between pre-existing primordia as in other phyllotactic systems. It is suggested that the number of carpel primordia formed is probably determined by the size of primordia relative to the floral apex, and the extent of continued growth of the floral apex. Luronium reinforces the concept that a form of trimery is fundamental for the Alismataceae up to the formation of three stamen pairs and adds to the possibilities for variation after this point. It is suggested for the Alismataceae in general that, according to taxon, trimerous development may be terminated at any point after the initiation of the stamen pairs, and after this the primordia are positioned individually in relation to pre-existing primordia. The switch from stamen to carpel initiation is not necessarily correlated with these phyllotactic changes.