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strap-shaped

Asteraceae

The family Asteraceae or Compositae (known as the aster, daisy, or sunflower family) is the largest family of flowering plants, in terms of number of species.

The name 'Asteraceae' is derived from the type genus Aster, while 'Compositae', an older but still valid name, 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.

According to the Royal Botanic Gardens of Kew, the family comprises more than 1,600 genera and 23,000 species. The largest genera are Senecio (1,500 species), Vernonia (1,000 species), Cousinia (600 species), Centaurea (600 species). The circumscription of the genera is often problematic and some of these have been frequently divided into minor subgroups.

Asteraceae are cosmopolitan, but most common in the temperate regions and tropical mountains.

Taxonomy

The family has been universally recognised and placed in the order Asterales.

Traditionally two subfamilies were recognised: Asteroideae (or 'Tubuliflorae') and Cichorioideae (or 'Liguliflorae'). The latter is paraphyletic and has been divided into many minor groups in most newer systems. The phylogenetic tree presented below is based on Panero & Funk (2002) and also shown in the APG system.

A tentative cladogram is shown below. The diamond denotes a very poorly supported branching (<50%), the dot a poorly supported branching (<80%). }}}}}}}}}}}}}}}}}}}}}}

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

Description

Asteraceae are most usually herbs, but some shrubs, trees and climbers do exist. They are generally easy to distinguish, mainly because of their characteristic inflorescence and many shared apomorphies.

Leaves and stems

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. 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 big single flower. This kind of flower-like inflorescences are quite widespread amongst plants and have 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 a specialised type of achene, sometimes called cypsela (plural cypselae). One seed per fruit is formed. It may sometimes be flat, winged or spiny and it adheres to the persistent pappus. Its morphology is often used to help determine plant relationships at the genus and species level. The seeds usually have little or lack endosperm.

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.

Ecology

Asteraceae are especially common in open and dry environments.

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 may have been within 42-36 million years, the stem group perhaps being up to 49 million years old.

It is still unknown whether the precise cause of their great success was the development of the calathid, 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.

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. Pollen from Ragweed Ambrosia is among the main causes of so called hay fever in the United States.

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. Notably 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

See also

Footnotes

References

  • Panero J. L. and V. A. Funk. 2002. Toward a phylogenetic subfamilial classification for the Compositae (Asteraceae) Proc. Biol. Soc. Wash. 115: 909-922.
  • Stevens, P. F. Angiosperm Phylogeny Website
  • Usher, George (1966). A dictionary of botany, including terms used in bio-chemistry, soil science, and statistics. Princeton: Van Nostrand.
  • Judd & al. ''Plant Systematics: A Phylogenetic Approach
  • http://ibot.sav.sk/icbn/main.htm International Code of Botanical Nomenclature (VIENNA CODE)
  • ITIS report 2002-09-10
  • Walters, Dirk R. and David J. Keil (1996). Vascular plant taxonomy. 4th ed. Kendall/Hunt Publishing Company. Dubuque, Iowa.
  • Wagner,W.L., D.R. Herbst, and S.H. Sohmer. 1990. Manual of the Flowering Plants of Hawai‘i, Vol. I. University of Hawaii Press, Honolulu. 988 pp.
  • D. J. N. Hind, C. Jeffrey & G. V. Pope (eds.), Advances in Compositae systematics. - Royal Bot. Gardens, Kew, 469 pp., 1995
  • Odom, Richard B.; William D. James, Timothy G. Berger (2000). Andrews' Diseases of the Skin: Clinical Dermatology. W.B. Saunders Company. ISBN 0721658326.

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