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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. The carpels (collectively the gynoecium, sometimes termed the fourth whorl) are borne in the centre of the flower, and normally terminate the floral axis.
This general pattern varies considerably among angiosperm groups, and many species possess unisexual flowers. The grass flower is typically subtended by a bract-like structure, the palea, that surrounds three (or, more commonly, two) reduced structures termed lodicules, which are normally interpreted as homologous with a single perianth whorl (probably the inner tepals) of other monocots.
Floral organ primordia are typically initiated from the outside inwards, in centripetal (acropetal) sequence towards the floral apex (Fig. 5.3). For example, in Drimys the innermost stamens are the last to be initiated in the developing bud, though they are the largest and the first to dehisce in the open flower113. However, in some species certain floral organs, either within a single whorl or in more than one whorl, are initiated in a different sequence or in groups (fascicles) (Fig. 5.4). For example, the stamens of some polyandrous palms are initiated centrifugally.
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