Evolutionary Study of Rheophytic Family Podostemaceae

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1. For elucidating the phylogeny and morphological evolution of the extremely specialized aquatic angiosperm family, field trips were made in Sri Lanka. India, Thailand. North America. Mexico, Guyana, and Brazil. By using materials that were collected during the trips, the following investigations were performed.2. A molecular phylogenetic analysis based on matK nucleotide sequences was made. The trees obtained show that the genus Weddellina, which has been referred to the subfamily Tristichoideae, is basal in the other subfamily Podostemoideae, indicating that the subfamily recognized so far is a paraphyletic taxon. The Torrenticola endemic to NE Australia is close to the Asian Cladopus. Although Dalzella and Indotristicha remarkably differ from each other morphologically, they are close to each other. Furthermore, the Old World genera examined make a cluster.3. Developmental morphological and anatomical studies were made of Malaccotristicha malayana the most basal genus in the subfamily Tristichoideae, and Hydrobryum japonicum, a most specialized genus in the subfamily Podostemoideae. The creeping axis of Malaccotristicha malayana is homologous with the root. The foliose Hydrobryum japonicum exhibits a peculiar, discontinuous development of the meristem, which gives rise to irregularly lobed thalli specific to Hydrobryum japonicum.