Flowers

Lena C Hileman, University of Kansas, Lawrence, Kansas, USA

Published online: August 2012

DOI: 10.1002/9780470015902.a0002063.pub2

Full Article on Wiley Online Library

Flowers are the reproductive structures of the flowering plants (angiosperms, class Magnoliopsida). The best evidence suggests that flowering plants arose in the Cretaceous, and that following their origin, rapid and expansive diversification in flower form ensued. Modern‐day flowers vary widely in size, symmetry, presence or absence and shape of floral organs, colour, colour patterning and arrangement of flowers in higher order inflorescence structures. This extensive diversity in flower form is strongly linked to the evolution of reproductive strategies, with many variations in flower form and colour representing adaptations that enhance reproductive success. These include adaptations that enhance cross‐pollination between individual plants, especially floral diversification that has resulted from an evolutionary history of plant–pollinator interaction.

Key Concepts:

A number of key characteristics distinguish the reproductive structures of flowering plants from reproductive structures of other seed plants.
Recent work in model flowering plant species has resulted in a deep understanding of the genetic mechanisms that control flowering and flower developmental patterning.
Following their origin in the Cretaceous, flowers diversified rapidly in size, shape and colour.
Flower diversification is largely the result of natural selection favouring modifications that enhance reproductive success through either self‐fertilisation or cross‐pollination.

Keywords: angiosperms; flower development; flowering plants; pollination; reproduction

	Figure 1. The structure of a typical flower (left), carpel (centre) and stamen (right).
	Figure 2. Structure of a typical embryo sac (female gametophyte, left) and pollen grain (male gametophyte, right).
	Figure 3. Each plant species has a particular flowering season that is triggered by a succession of long days, or short days; species with flowering not triggered by day length are day neutral. Blue on each clock indicates hours of daylight, yellow indicates hours of dark, and the white sector in the clock on the right indicates a pulse of light in the middle of dark period, which converts one long dark period to two short dark periods resulting in flowering of long‐day (short night) plants.
	Figure 4. Patterns of gene expression during flower development. (Left) A‐, B‐ and C‐class genes function in a combinatorial manner to specify the identity of floral organs during development. In the outer whorl of the flower, A‐class gene products function in the absence of B‐ and C‐class genes resulting in the development of sepals. In the second whorl, A‐ and B‐class gene products function together in the absence of C‐class genes resulting in the development of petals. In the third whorl, B‐ and C‐class gene products function together in the absence of A‐class genes resulting in the development of stamens. And in the fourth (centre) whorl of the flower, C‐class gene products function in the absence of A‐ and B‐class genes resulting in the development of carpels. (Right) Bilateral flower symmetry can be determined by differential expression of regulatory genes on one side of the flower, but not on the other side. Illustrated here is the expression of a gene that signals differentiation of the dorsal (adaxial) side of the flower differently from the ventral (abaxial) side of the flower. The CYCLOIDEA gene in snapdragon is expressed similarly on the dorsal side of developing flowers.
	Figure 5. Inflorescences. (a) Determinate inflorescences, or cymes, include monochasia and dichasia. Different forms of monochasia produce different arrangements of flowers when viewed from above. (b) Indeterminate inflorescences or racemes. The spike and spadix are two forms of raceme. (c) The umbel can be derived from both cyme and raceme.

References
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Further Reading
	book Bell AD (1991) Plant Form. An Illustrated Guide to Flowering Plant Morphology. Oxford: Oxford University Press.
	book Endress PK (1994) Diversity and Evolutionary Biology of Tropical Flowers. Cambridge: Cambridge University Press.
	book Friis EM, Chaloner WG and Crane PR (eds) (1989) The Origins of Angiosperms and Their Biological Consequences. Cambridge: Cambridge University Press.
	book Glover B (2007) Understanding Flowers and Flowering, an Integrated Approach. Oxford: Oxford University Press.
	book Grant V (1975) Genetics of Flowering Plants. New York: Columbia University Press.
	book Heywood VH (ed.) (1978) Flowering Plants of the World. Oxford: Oxford University Press.
	Krizek BA and Fletcher JC (2005) Molecular mechanisms of flower development: an armchair guide. Nature Reviews. Genetics 6: 688–698.
	Meyerowitz EM (1994) The genetics of flower development. Scientific American 271: 56–65.
	book Proctor M, Yeo P and Lack A (1996) The Natural History of Pollination. London: Harper Collins.
	book Stebbins GL (1974) Flowering Plants. Cambridge, MA: The Belknap Press of Harvard University Press.

Article Title:	Flowers
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Flowers

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