title: Floral anatomy and vegetative development in Ceratophyllum demersum: a morphological picture of an "unsolved" plant.
abstract: OBJECTIVE
  The phylogenetic position of Ceratophyllum is still controversial in recent molecular analyses of angiosperms, with various suggestions of a sister group relation to all other angiosperms, eudicots, monocots, eudicots + monocots, and magnoliids. Therefore, the morphological characters of Ceratophyllum are important for resolving the phylogeny of angiosperms. In this study, we observed the detailed developmental anatomy of all lateral organs and their configurations to elucidate the floral development and phyllotactic pattern of Ceratophyllum demersum.
  
  
  METHODS
  We observed fixed shoots of C. demersum with scanning electron microscopy and serial sections of the samples with light microscopy.
  
  
  RESULTS
  Bract primordia arise first, followed by the stamen primordia in staminate flowers. Both bracts and stamens initiate unidirectionally, first on the abaxial side of the floral apex and later on the adaxial side, most likely due to the contact pressure imposed by the leaf primordium at the superior node. In pistillate flowers, bract primordia on the abaxial side were also initiated first. The configuration of buds at one node showed six patterns and each pattern included at least one vegetative bud, and flower buds were always accompanied by vegetative buds at the same node.
  
  
  CONCLUSIONS
  The initiation pattern of organs in the outer whorls of C. demersum flowers is distorted by mechanical pressure, resulting in the phyllotactic variation of staminate flowers. Vegetative buds are the main axillary buds with floral buds as accessory buds, which suggests that the shoot of C. demersum has been modified from a decussate phyllotaxis.
authors.affiliation: Department of Biology, Tokyo Gakugei University, 4-1-1 Nukui Kita-machi, Koganei-shi, Tokyo 184-8501, Japan akitoshi@u-gakugei.ac.jp.
authors.forename: Akitoshi
authors.initials: A
authors.lastname: Iwamoto
authors.affiliation: Department of Biology, Tokyo Gakugei University, 4-1-1 Nukui Kita-machi, Koganei-shi, Tokyo 184-8501, Japan.
authors.forename: Ryoko
authors.initials: R
authors.lastname: Izumidate
authors.affiliation: Royal Botanic Garden Edinburgh, 20A Inverleith Row, Edinburgh EH3 5LR, UK.
authors.forename: Louis P
authors.initials: LP
authors.lastname: Ronse De Craene
mesh_terms: D035264:Flowers / Q000033:anatomy & histology / Q000254:growth & development / Q000648:ultrastructure; D019684:Magnoliopsida / Q000033:anatomy & histology* / Q000254:growth & development* / Q000648:ultrastructure; D008855:Microscopy, Electron, Scanning; D018520:Plant Shoots / Q000033:anatomy & histology / Q000254:growth & development / Q000648:ultrastructure
keywords: Ceratophyllaceae; Ceratophyllum demersum; anisoclady; basal angiosperm; floral anatomy; mechanical force; phyllotaxis; vegetative development