Floral Organs

Flowers are complex structures that consist of several organ types borne on a central axis (the receptacle). In many species each flower is subtended by a modified leaf-like structure termed a bract (Fig. 1.2), though bracts are absent from some other species. Within each flower, the organs are arranged in distinct bands (whorls) or in a spiral pattern (Figs 5.1, 5.2). The degree of fusion of individual floral organs within each flower is normally characteristic of a species (i.e. genetically determined). Fusion between similar organ types borne in the same whorl is termed connation. Fusion between different organ types borne in adjacent whorls is termed adnation.

The outer two types of floral organs (collectively the perianth) are modified leaf-like structures, termed sepals (collectively the calyx, or sometimes the first whorl) and petals (collectively the corolla, or the second whorl). In many monocots and magnoliids the perianth organs are morphologically indistinguishable from each other, and are collectively termed tepals, rather than differentiated into sepals and petals. Enclosed within the perianth are the stamens, which are collectively termed the androecium, or sometimes the third whorl, though they are often borne in two or more distinct whorls.