title: Towards an ontogenetic understanding of inflorescence diversity.
abstract: OBJECTIVE
  Conceptual and terminological conflicts in inflorescence morphology indicate a lack of understanding of the phenotypic diversity of inflorescences. In this study, an ontogeny-based inflorescence concept is presented considering different meristem types and developmental pathways. By going back to the ontogenetic origin, diversity is reduced to a limited number of types and terms.
  
  
  METHODS
  Species from 105 genera in 52 angiosperm families are investigated to identify their specific reproductive meristems and developmental pathways. Based on these studies, long-term experience with inflorescences and literature research, a conceptual framework for the understanding of inflorescences is presented.
  
  
  RESULTS
  Ontogeny reveals that reproductive systems traditionally called inflorescences fall into three groups, i.e. 'flowering shoot systems' (FSS), 'inflorescences' sensu stricto and 'floral units' (FUs). Our concept is, first, based on the identification of reproductive meristem position and developmental potential. The FSS, defined as a seasonal growth unit, is used as a reference framework. As the FSS is a leafy shoot system bearing reproductive units, foliage and flowering sequence play an important role. Second, the identification of two different flower-producing meristems is essential. While 'inflorescence meristems' (IMs) share acropetal primordia production with vegetative meristems, 'floral unit meristems' (FUMs) resemble flower meristems in being indeterminate. IMs produce the basic inflorescence types, i.e. compound and simple racemes, panicles and botryoids. FUMs give rise to dense, often flower-like units (e.g. heads). They occur solitarily at the FSS or occupy flower positions in inflorescences, rendering the latter thyrses in the case of cymose branching.
  
  
  CONCLUSIONS
  The ontogenetic concept differs from all existing inflorescence concepts in being based on meristems and developmental processes. It includes clear terms and allows homology statements. Transitional forms are an explicit part of the concept, illustrating the ontogenetic potential for character transformation in evolution.
authors: Institut für Spezielle Botanik und Botanischer Garten, Johannes-Gutenberg Universität Mainz, Germany.
authors: Regine
authors: R
authors: Claßen-Bockhoff
authors: Kester
authors: K
authors: Bull-Hereñu
mesh_terms: D044822:Biodiversity; D049109:Cell Proliferation; D056626:Inflorescence / Q000033:anatomy & histology / Q000166:cytology / Q000254:growth & development* / Q000648:ultrastructure; D019684:Magnoliopsida / Q000033:anatomy & histology / Q000166:cytology / Q000254:growth & development* / Q000648:ultrastructure; D018519:Meristem / Q000033:anatomy & histology / Q000166:cytology / Q000254:growth & development / Q000648:ultrastructure
keywords: Botryoid; floral unit (FU); flowering sequence; flowering shoot system (FSS); foliage; inflorescence; meristem potential; ontogenetic concept; panicle; position; raceme; thyrse