Pseudanthium
A pseudanthium (Greek for "false flower") or flower head is a special type of inflorescence[1], in which several flowers are grouped together to form a flower-like structure. The real flowers are generally small and greatly reduced, but can sometimes be quite large (as in the sunflower flower-head). Pseudanthia take various forms.
Pseudanthia are characteristic of the sunflower family (Asteraceae), where they are called calathidia, calathids or capitula. The flowers are differentiated into ray and disk flowers, which are unique to the family. The disk flowers in the center of the pseudanthium (called a flower head or capitulum in Asteraceae) are actinomorphic and the corolla is fused into a tube. Flowers on the periphery are zygomorphic and the corolla has one large lobe (the so-called "petals" of a daisy are individual ray flowers, for example). Either ray or disk flowers may be absent in some plants: Senecio vulgaris lacks ray flowers[2][3] and Taraxacum officinale lacks disk flowers.[4][5] The capitulum has a whorl of bracts below the flowers, forming an involucre.
In all cases, a pseudanthium (inflorescence) is superficially indistinguishable from a flower, but closer inspection of its anatomy will reveal that it is composed of multiple flowers. Thus, the pseudanthium represents an evolutionary convergence of the inflorescence to a reduced reproductive unit that may function in pollination like a single flower, at least in plants that are animal pollinated.
Pseudanthia occur in the following plant families:
- Apiaceae
- Araceae
- Asteraceae
- Campanulaceae
- Centrolepidaceae
- Cornaceae
- Cyperaceae
- Dipsacaceae
- Euphorbiaceae — there the pseudanthia are called cyathia
- Eriocaulaceae
- Hamamelidaceae
- Moraceae
- Poaceae
- Pontederiaceae — in Hydrothrix
- Proteaceae
- Rubiaceae
- Saururaceae — in Anemopsis
In some families it is not yet clear whether the 'flower' represents a pseudanthium, because the anatomical work has not been done (or is still ambiguous due to considerable evolutionary reduction). Possible pseudanthia of this type may occur in the following families:
[edit] Gallery
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Diagram of a flower head. Note bracts surrounding the flowers, which would be absent on a capitulum.
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Flower head of a common daisy (Bellis perennis)
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Flowers open in succession in head of a sunflower (Helianthus annuus), with ray florets forming the 'petals'
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Close up of the ray corolla of a Hieracium lachenalii; every "petal" is actually a separate five-petaled flower complete with its own stamens and making its own fruit.
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Discoid (having only disk flowers) flower heads of a rubber rabbitbrush.
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[edit] References
- ^ Hutchinson, John (1964). The genera of flowering plants (Angiospermae). Oxford: Clarendon Press. LCCN 65-676.
- ^ http://www.missouriplants.com/Yellowalt/Senecio_vulgaris_page.html
- ^ http://www.cdfa.ca.gov/phpps/IPC/weedinfo/senecio.htm
- ^ http://www.floridata.com/ref/T/tara_off.cfm
- ^ http://www.missouriplants.com/Yellowalt/Taraxacum_erythrospermum_page.html
- ^ Petra Hoffmann, Hashendra S. Kathriarachchi, and Kenneth J. Wurdack. 2006. "A Phylogenetic Classification of Phyllanthaceae (Malpighiales)." Kew Bulletin. 61(1):40.
