title: Seedling diversity in Hydatellaceae: implications for the evolution of angiosperm cotyledons.
abstract: OBJECTIVE
  Cotyledon number has long been a primary morphological feature distinguishing monocots from other angiosperms. Recent placement of Hydatellaceae near the early-divergent angiosperm order Nymphaeales, rather than in the monocot order Poales, has prompted reassessment of seedling morphology in this poorly known family.
  
  
  METHODS
  Seedlings of six species representing all eco-geographical groups of Hydatellaceae are described using light and scanning electron microscopy.
  
  
  RESULTS
  Two seedling types were discovered. Material examined of Trithuria submersa, T. bibracteata, T. austinensis and T. filamentosa possess a transparent bilobed sheathing structure that surrounds the main axis below the first foliage leaf. The seed coat is attached to the sheathing structure. Seedlings of Trithuria lanterna and T. konkanensis lack a sheathing structure, and the seed coat is attached to a short, narrow lateral outgrowth on the main axis of the seedling.
  
  
  CONCLUSIONS
  The sheathing structure that is present in seedlings of some Hydatellaceae could be homologized with the two united cotyledons of water lilies. It also resembles the single cotyledon of some monocots, and hence demonstrates a possible pathway of the origin of a monocot-like embryo, though no homology is implied. The sheathing structure is reduced in Trithuria lanterna and T. konkanensis, and the short, narrow outgrowth of its seedling could represent a single cotyledon. This synapomorphy suggests that the only Indian species of Hydatellaceae, T. konkanensis, is closer to the northern Australian T. lanterna than to the south-western Australian T. bibracteata.
authors.affiliation: Biological Faculty, Moscow State University, 119991 Moscow, Russia.
authors.forename: Dmitry D
authors.initials: DD
authors.lastname: Sokoloff
authors.forename: Margarita V
authors.initials: MV
authors.lastname: Remizowa
authors.forename: Terry D
authors.initials: TD
authors.lastname: Macfarlane
authors.forename: Renee E
authors.initials: RE
authors.lastname: Tuckett
authors.forename: Margaret M
authors.initials: MM
authors.lastname: Ramsay
authors.forename: Anton S
authors.initials: AS
authors.lastname: Beer
authors.forename: Shrirang R
authors.initials: SR
authors.lastname: Yadav
authors.forename: Paula J
authors.initials: PJ
authors.lastname: Rudall
mesh_terms: D005075:Biological Evolution; D018548:Cotyledon / Q000145:classification / Q000254:growth & development / Q000648:ultrastructure*; D019684:Magnoliopsida / Q000145:classification* / Q000254:growth & development / Q000648:ultrastructure; D008855:Microscopy, Electron, Scanning; D018515:Plant Leaves / Q000254:growth & development / Q000648:ultrastructure; D018517:Plant Roots / Q000254:growth & development / Q000648:ultrastructure; D036226:Seedlings / Q000145:classification* / Q000254:growth & development / Q000648:ultrastructure; D012639:Seeds / Q000254:growth & development / Q000648:ultrastructure