A developmental study of pollen dyads and notes on floral development inScheuchzeria(Alismatales: Scheuchzeriaceae)

Abstract

Scheuchzeria palustris, the only member of Scheuchzeriaceae, is unique among Alismatales in several characters, including flower-subtending bracts with conspicuous laminas and pollen dispersed in permanent dyads. Earlier studies revealed unidirectional flower development in some monocots with massive flower-subtending bracts, but not in Scheuchzeria. Permanent pollen or spore dyads are extremely rare among extant land plants and in no case have they been studied developmentally. The paper provides new data on flower development and the first description of pollen development in Scheuchzeria. Flowers are arranged in a closed raceme. Development of lateral flowers is unidirectional, with delayed initiation of abaxial tepals and stamens. Flowers are often trimerous and pentacyclic, but numerous deviations from this groundplan are observed. Dyad development is investigated from microspore mother cells to mature pollen and their germination by electron microscopy and cytochemical methods. Meiosis is successive. Tetrads are irregular (42%), tetragonal (35%), decussate (13%), linear (7%) and T-shaped (3%). During the tetrad period, the microspores are surrounded by a callose special wall of different thickness. Disintegration of tetrads into two dyads takes place along the plane of the first cytokinesis, where the thickness of the callose wall is maximal. Pollen grains are held together in the dyads due to a simple fusion of tectal layers, this fusion takes place at the late tetrad period. Developmental data did not reveal any possible aperture sites in Scheuchzeria. This favours the hypothesis of complete loss and subsequent regain of apertures in the course of evolution of the ‘tepaloid clade’ of Alismatales.