Definition of flowers

Flowers were defined in many different ways in the past and there is no general agreement about how a flower should be defined. In the past there was emphasis on two main contrasting hypotheses (reviewed in Bateman, Hilton and Rudall, 2006). The pseudanthial hypothesis accepts that flowers evolved from a branched, multiaxial structure, i.e. a condensed compound inflorescence (e.g. Eichler, 1875; Eames, 1961). This means that a flower is an assemblage of separately functioning entities that became grouped together. The euanthium hypothesis stated that the flower evolved from a simple uniaxial (euanthial) structure, i.e. a condensed sporophyll-bearing axis with proximal microsporophylls and distal megasporophylls (e.g. Arber and Parkin, 1907). Floral organs all have attributes of leaves, and leaf-like elements, such as stipules, leaf bases, petioles and blades, occasionally appear in flowers (e.g. Guédès, 1979). More recently, phylogenetic studies have supported the theory that flowers evolved once and that all flowers are thus homologous (‘anthophyte hypothesis’ reviewed in Bateman, Hilton and Rudall, 2006). The theory is supported by evolutionary developmental evidence that the same genes are acting on the flower and vegetative shoot, and that the flower is best interpreted as a short shoot with specialized leaves (Glover, 2007).

More specifically, a flower can be defined a determinate structure with a generally defined number of organs; it bears both staminate and pistillate parts and organs are set in four series: sepals, petals, stamens and carpels.