Pseudanthium · Lablypedia

Pseudanthia are characteristic of the daisy and sunflower family (Asteraceae), whose flowers are differentiated into ray flowers and disk flowers, unique to this family. The disk flowers in the center of the pseudanthium 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 and Taraxacum officinale lacks disk flowers. The individual flowers of a pseudanthium in the family Asteraceae (or Compositae) are commonly called florets. The pseudanthium has a whorl of bracts below the flowers, forming an involucre.

In all cases, a pseudanthium 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 may be grouped into types. The first type has units of individual flowers that are recognizable as single flowers even if fused. In the second type, the flowers do not appear as individual units and certain organs like stamens and carpels cannot be associated with any individual flowers.

History

The term pseudanthium was originally applied to flowers with stamens in two whorls with the outer whorl opposite the petals (obdiplostemonate) or polyandric flowers; by the early 1900s the term was repurposed by the advocates of the 'pseudanthium theory' which assumed flower evolution originated from a polyaxial instead of a monoaxial configuration.

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Synorganization

The collection of independent organs into a complex structure is called synorganization.

Head

Head is an equivalent term for flower head and pseudanthium when used in the botanical sense.

Capitulum

Capitulum (plural capitula) can be used as an exact synonym for pseudanthium and flower head; however, this use is generally but not always restricted to the family Asteraceae. At least one source defines it as a small flower head. In addition to its botanical use as a term meaning flower head it is also used to mean the top of the sphagnum plant.

Calathid

Calathid (plural calathids or calathidia) is a very rarely used term. It was defined in the 1966 book The genera of flowering plants (Angiospermae) as a specific term for a flower head of a plant in the family Asteraceae. However, on-line botanical glossaries do not define it, and Google Scholar does not link to any significant usage of the term in a botanical sense.

Plant families

Pseudanthia occur in 40 plant families including:

Adoxaceae — in some Viburnum spp.
Apiaceae — pseudanthia are called umbels
Araceae — pseudanthia are called spadices
Asteraceae — The capitula (singular capitulum) or flower heads, which are collections of different types of flowers, is a pseudanthium. The individual flowers of a capitulum are called florets. Commonly the capitulum has ray flowers specialized to attract pollinators arranged surrounding disc flowers responsible for sexual reproduction, perianth symmetry can be variable within the family.
Campanulaceae
Centrolepidaceae — Where individual male and female flowers are grouped together and wrapped in bracts forming a pseudanthium appearing as a bisexual flower.
Cornaceae

Cyperaceae — In subfamily Mapanioideae, pseudanthia are termed spicoids. In Lepironia sp the pseudanthium is greatly condensed with staminate flowers surrounding a central terminal pistillate female flower.
Dipsacaceae
Euphorbiaceae — in Euphorbia — pseudanthia are called cyathia, composed of a single carpal flower with few to many single-stamen staminate flowers contained within a cup-shaped structure or bracts; the bracts are often rimmed with nectaries and less commonly petal-like structures. The central cyathia may be composed of all male flowers.

Eriocaulaceae
Hamamelidaceae — in Rhodoleia
Marcgraviaceae
Moraceae

Myrtaceae — in Actinodium — the pseudanthia is a head-like structure with fertile flowers in the center and showy ray-like structures along the outside.
Nothofagaceae — in subgenus Lophozonia — a three‐flowered dichasium without branches.
Nyssaceae — in Davidia
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:

Araceae — in subfamily Lemnoideae
Hydatellaceae
Pandanaceae
Phyllanthaceae
Triuridaceae