Asteraceae

The Asteraceae or Compositae, also referred to as the aster, daisy, or sunflower family, is the largest family of vascular plants.^[4] The family has more than 22,750 currently accepted species, spread across 1620 genera, and 12 subfamilies.^[4][5] The largest genera are Senecio (1,500 species), Vernonia (1,000 species), Cousinia (600 species) and Centaurea (600 species).

Most members of the Asteraceae are herbaceous, but a significant number are also shrubs, vines and trees. The family is distributed throughout the world, and is most common in the arid and semi-arid regions of subtropical and lower temperate latitudes.^[6]

Many economically important products come from composites, including cooking oils, lettuce, sunflower seeds, artichokes, sweetening agents, and teas. Several genera are also very popular with the horticultural community, these include marigolds, chrysanthemums, dahlias, zinnias, and heleniums.

Contents 1 Etymology 2 Distribution 3 Taxonomy 4 Characteristics 4.1 Roots and Stems 4.2 Leaves 4.3 Flowers 4.4 Fruits and Seeds 4.5 Metabolites 5 Ecology 6 Evolution 7 Uses 8 Genera 9 Image gallery 10 See also 11 References 12 External links

Etymology

The name Asteraceae is derived from the type genus Aster, which is a Greek term, meaning "star".^[7] While Compositae, an older but still valid name,^[8] means composite and refers to the characteristic inflorescence, a special type of pseudanthium found in only a few other angiosperm families. The study of this family is known as synantherology.

Distribution

Daisies have a worldwide distribution, being found everywhere except Antarctica. They are especially numerous in tropical and subtropical regions (notably Central America, eastern Brazil, the Andes, the Mediterranean, southern Africa, central Asia, and southwestern China).^[5]

Taxonomy

The Compositae was first described in 1792 by the German botanist Paul Dietrich Giseke.^[9] Traditionally two subfamilies were recognised: Asteroideae (or Tubuliflorae) and Cichorioideae (or Liguliflorae). The latter has been shown to be extensively paraphyletic, and has now been divided into 11 subfamilies, but the former still stands. The phylogenetic tree presented below is based on Panero & Funk (2002).^[10] The diamond denotes a very poorly supported node (<50% bootstrap support), the dot a poorly supported node (<80%).^[4]

Barnadesioideae: 9 genera, 93 species. South America, mainly the Andes. Stifftioideae: South America and Asia. ♦ Mutisioideae: 58 genera, 750 species. South America. • Wunderlichioideae: 8 genera, 24 species, mostly in Venezuela and Guyana Gochnatioideae: 4 or 5 genera, 90 species. Hecastocleidoideae: Only Hecastocleis shockleyi. South-western US. Carduoideae: 83 genera, 2,500 species. Worldwide. Pertyoideae: 5 or 6 genera, 70 species. Gymnarrhenoideae: Only Gymnarrhena micrantha. Northern Africa. • Cichorioideae: 224 genera, 3,200 species. Worldwide. Corymbioideae: Only the genus Corymbium, with 7 species. • Asteroideae: 1,130 genera and 16,200 species. Worldwide.

It is noteworthy that the four subfamilies Asteroideae, Cichorioideae, Carduoideae and Mutisioideae comprise 99% of the specific diversity of the whole family (approximately 70%, 14%, 11% and 3% respectively).

Because of the morphological complexity exhibited by this family, agreeing on appropriate generic circumscriptions has often been difficult for Asteraceae taxonomists. As a result several of these genera have required multiple revisions.^[11]

Characteristics

Asteraceae are mostly herbaceous plants, but some shrubs, trees and climbers do exist. Daisies are generally easy to distinguish from other plants, mainly because of their characteristic inflorescence and many shared apomorphies.^[11] However, many closely related species in this family are notoriously difficult to identify to the species level.

Roots and Stems

Daisy roots are usually taproots, and sometimes fibrous. Stems are generally erect, but sometimes prostrate to ascending. Some species have underground stems in the form of caudices or rhizomes, these can be fleshy or woody depending on the species.^[6]

Leaves

The leaves and the stems very often contain secretory canals with resin or latex (particularly common among the Cichorioideae). The leaves can be alternate, opposite, or whorled. They may be simple, but are often deeply lobed or otherwise incised, often conduplicate or revolute. The margins can be entire or dentate.

Flowers

The most evident characteristic of Asteraceae is perhaps their inflorescence: a specialised capitulum, technically called a calathid or calathidium, but generally referred to as flower head or, alternatively, simply capitulum.^[12] The capitulum is a contracted raceme composed of numerous individual sessile flowers, called the florets, all sharing the same receptacle.

The capitulum of the Asteraceae has evolved many characteristics that make it look superficially like a single flower. This type of flower-like inflorescence is fairly widespread amongst angiosperms, and has been given the name of pseudanthia.

Many bracts form an involucre under the basis of the capitulum; these are called "phyllaries", or "involucral bracts". They may simulate the sepals of the pseudanthium. These are mostly herbaceous but can also be brightly coloured (e.g. Helichrysum) or have a scarious texture. The bracts can be free or fused, and arranged in one to many rows, overlapping like the tiles of a roof (imbricate) or not (this variation is important in identification of tribes and genera).

Each floret may itself be subtended by a bract, called a "palea" or "receptacular bract". These bracts as a group are often called "chaff". The presence or absence of these bracts, their distribution on the receptacle, and their size and shape are all important diagnostic characteristics for genera and tribes.

The florets have five petals fused at the base to form a corolla tube and they may be either actinomorphic or zygomorphic. Disc florets are usually actinomorphic, with five petal lips on the rim of the corolla tube. The petal lips may be either very short, or long, in which case they form deeply lobed petals. The latter is the only kind of floret in the Carduoideae, while the first kind is more widespread. Ray florets are always highly zygomorphic and are characterised by the presence of a ligule, a strap-shaped structure on the edge of the corolla tube consisting of fused petals. In the Asteroideae and other minor subfamilies these are usually borne only on florets at the circumference of the capitulum and have a 3+2 scheme – above the fused corolla tube, three very long fused petals form the ligule, with the other two petals being inconspicuously small. The Cichorioidea has only ray florets, with a 5+0 scheme – all five petals form the ligule. A 4+1 scheme is found in the Barnadesioideae. The tip of the ligule is often divided into teeth, each one representing a petal. Some marginal florets may have no petals at all (filiform floret).

The calyx of the florets may be absent, but when present, it is always modified into a pappus of two or more teeth, scales or bristles and this is often involved in the dispersion of the seeds. As with the bracts, the nature of the pappus is an important diagnostic feature.

There are usually five stamens. The filaments are fused to the corolla, while the anthers are generally connate (syngenesious anthers), thus forming a sort of tube around the style (theca). They commonly have basal and/or apical appendages. Pollen is released inside the tube and is collected around the growing style, expelled with a sort of pump mechanism (nüdelspritze) or a brush.

The pistil is made of two connate carpels. The style has two lobes; stigmatic tissue may be located in the interior surface or form two lateral lines. The ovary is inferior and has only one ovule, with basal placentation.

Fruits and Seeds

The fruit of the Asteraceae is achene-like, and is called a cypsela (plural cypselae). Although there are two fused carpels, there is only one locule, and only one seed per fruit is formed. It may sometimes be winged or spiny because the pappus, which is derived from calyx tissue often remains on the fruit (for example in dandelion). In some species, however, the pappus falls off (for example in Helianthus). Cypsela morphology is often used to help determine plant relationships at the genus and species level.^[13] The mature seeds usually have little endosperm or none.^[11]

Metabolites

Asteraceae generally store energy in the form of inulin. They produce iso/chlorogenic acid, sesquiterpene lactones, pentacyclic triterpene alcohols, various alkaloids, acetylenes (cyclic, aromatic, with vinyl end groups), tannins. They have terpenoid essential oils which never contain iridoids.^[4]

Ecology

Asteraceae are especially common in open and dry environments.^[11]

Many members of the Asteraceae are pollinated by insects, which explains their value in attracting beneficial insects, but anemophyly is also present (e.g. Ambrosia, Artemisia). There are many apomictic species in the family.

Seeds are ordinarily dispersed intact with the fruiting body, the cypsela. Wind dispersal is common (anemochory) assisted by a hairy pappus. Another common variation is epizoochory, in which the dispersal unit, a single cypsela (e.g. Bidens) or entire capitulum (e.g. Arctium) provided with hooks, spines or some equivalent structure, sticks to the fur or plumage of an animal (or even to clothes, like in the photo) just to fall off later far from its mother plant.

Evolution

Diversification of Asteraceae appears to have taken place roughly 42-36 million years ago, the stem group perhaps being up to 49 million years old.^[4]

It is still unknown whether the precise cause of their great success was the development of the highly specialised capitulum, their ability to store energy as fructans (mainly inulin), which is an advantage in relatively dry zones, or some combination of these and possibly other factors.^[4]

Uses

Commercially important plants in the Asteraceae include the food crops Lactuca sativa (lettuce), Cichorium (chicory), Cynara scolymus (globe artichoke), Helianthus annuus (sunflower), Smallanthus sonchifolius (yacón), Carthamus tinctorius (safflower) and Helianthus tuberosus (Jerusalem artichoke).

Other commercially important species include Compositae used as herbs and in herbal teas and other beverages. Chamomile, which comes from two different species, the annual Matricaria recutita or German chamomile, and the perennial Chamaemelum nobile, also called Roman chamomile. Calendula, also called the pot marigold is grown commercially for herbal teas and the potpourri industry. Echinacea (Echinacea purpurea), used as a medicinal tea. Winter tarragon, also called Mexican mint marigold, Tagetes lucida is commonly grown and used as a tarragon substitute in climates where tarragon will not survive. Finally, the wormwood genus Artemisia includes absinthe (A. absinthium) and tarragon (A. dracunculus).

Industrial use of Compositae is also known. Common in all commercial poultry feed, marigold (Tagetes patula) is grown primarily in Mexico. Marigold oil, extracted from Tagetes minuta is used by the metric ton in the cola and cigarette industry.

Plants in Asteraceae are medically important in areas that don't have access to Western medicine. They are also commonly featured in medical and phytochemical journals because the sesquiterpene lactone compounds contained within them are an important cause of allergic contact dermatitis. Allergy to these compounds is the leading cause of allergic contact dermatitis in florists in the US.^[14] Pollen from ragweed Ambrosia is among the main causes of so called hay fever in the United States.^[15]

Many members of the family are grown as ornamental plants for their flowers and some are important ornamental crops for the cut flower industry. Some examples are Chrysanthemum, Gerbera, Calendula, Dendranthema, Argyranthemum, Dahlia, Tagetes, Zinnia and many others.

Many members of Asteraceae are copious nectar producers and are useful for evaluating pollinator populations during their bloom. Centaurea (knapweed), Helianthus annuus (domestic sunflower), and some species of Solidago (goldenrod) are major "honey plants" for beekeepers. Solidago produces relatively high protein pollen, which helps honey bees overwinter.

Some members of the Asteraceae are economically important as weeds. Notable in the United States are the ragwort, Senecio jacobaea, groundsel Senecio vulgaris and Taraxacum (dandelion).

The genera Tanacetum, Chrysanthemum and Pulicaria contain species with insecticidal properties.

Parthenium argentatum (guayule) is a source of hypoallergenic latex.

Genera

Image gallery

Acanthospermum hispidum	Bidens biternata	Carduus tenuiflorus (Carduoideae)	Vernonia cinerea with its pollinator bee	Eclipta prostrata	Eupatorium cannabinum
Gerbera sp. (Mutisoideae). Note the zygomorphic disc flowers.	Grangea maderaspatana	Pertya scandens (Pertyoideae)	Cichorium intybus (Cichorioideae). Note the dark blue anther tubes and the bilabiate styles.	Aster novae-angliae (Asteroideae)	Ray floret (as in Cichorioideae, and the outer florets in Asteroideae)
Disc floret (as in Asteroideae)	The involucral leaves of the capitula of the globe artichoke (here flowering) are eaten as vegetable	The large seed head of Tragopogon pratensis is composed of many single achenes, each with its own pappus	Sticking teeth of a Bidens tripartita achene under stereomicroscope (30X).	The stem of the dandelion (Taraxacum, Cichorioideae) contains a bitter latex.

See also

• Terminology for Asteraceae
• Damned yellow composite

References

External links

• Asteraceae at the Angiosperm Phylogeny Website
• Asteraceae at the Tree of Life Web Project
• Compositae at The Families of Flowering Plants (DELTA)
• Asteraceae at the online Flora of North America
• Asteraceae at the online Flora of Pakistan
• Asteraceae at the online Flora of Zimbabwe
• Asteraceae at the online Flora of Australia
• Compositae at the online Flora of New Zealand