Asteraceae
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Asteraceae (Template:IPAc-en) is a large family of flowering plants that consists of over 32,000 known species in over 1,900 genera within the order Asterales. The number of species in Asteraceae is rivaled only by the Orchidaceae, and which is the larger family is unclear as the quantity of extant species in each family is unknown. The Asteraceae were first described in the year 1740 and given the original name Compositae.<ref name=BarkleyEtAl/> The family is commonly known as the aster, daisy, composite, or sunflower family.
Most species of Asteraceae are herbaceous plants, and may be annual, biennial, or perennial, but there are also shrubs, vines, and trees. The family has a widespread distribution, from subpolar to tropical regions, in a wide variety of habitats. Most occur in hot desert and cold or hot semi-desert climates, and they are found on every continent but Antarctica. Their common primary characteristic is compound flower heads, technically known as capitula, consisting of sometimes hundreds of tiny individual florets enclosed by a whorl of protective involucral bracts.
The oldest known fossils are pollen grains from the Late Cretaceous (Campanian to Maastrichtian) of Antarctica, dated to Template:Circa million years ago (mya). It is estimated that the crown group of Asteraceae evolved at least 85.9 mya (Late Cretaceous, Santonian) with a stem node age of 88–89 mya (Late Cretaceous, Coniacian).
Asteraceae is an economically important family, providing food staples, garden plants, and herbal medicines. Species outside of their native ranges can become weedy or invasive.
DescriptionEdit
Members of the Asteraceae are mostly herbaceous plants, but some shrubs, vines, and trees (such as Lachanodes arborea) do exist. Asteraceae species are generally easy to distinguish from other plants because of their unique inflorescence and other shared characteristics, such as the joined anthers of the stamens.<ref name=Judd/> Nonetheless, determining genera and species of some groups such as Hieracium is notoriously difficult (see "damned yellow composite" for example).<ref name=MandelEtAl2019 />
RootsEdit
Members of the family Asteraceae generally produce taproots, but sometimes they possess fibrous root systems. Some species have underground stems in the form of caudices or rhizomes. These can be fleshy or woody depending on the species.<ref name=BarkleyEtAl/>
StemsEdit
The stems are herbaceous, aerial, branched, and cylindrical with glandular hairs, usually erect, but can be prostrate to ascending. The stems can contain secretory canals with resin,<ref name=BarkleyEtAl/> or latex, which is particularly common among the Cichorioideae.<ref name=KGL/>
LeavesEdit
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 also can be entire or toothed. Resin<ref name=BarkleyEtAl/> or latex<ref name=KGL/> can also be present in the leaves.
InflorescencesEdit
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Nearly all Asteraceae bear their flowers in dense flower heads called capitula. They are surrounded by involucral bracts, and when viewed from a distance, each capitulum may appear to be a single flower. Enlarged outer (peripheral) flowers in the capitulum may resemble petals, and the involucral bracts may look like a calyx.<ref>Template:Cite journal</ref> Notable exceptions include Hecastocleis shockleyi (the only species in the subfamily Hecastocleidoideae)<ref name=EoL>{{#invoke:citation/CS1|citation |CitationClass=web }} and Template:Cite book</ref> and the species of the genus Corymbium (the only genus in the subfamily Corymbioideae),<ref name= Weitz>Template:Cite journal</ref> which have one-flowered bisexual capitulas, Gundelia with one-flowered unisexual capitulas,<ref>Template:Cite book cited on {{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and Gymnarrhena micrantha with one-flowered female capitulas and few flowered male capitulas.<ref name=FFS>Template:Cite book</ref>
Floral headsEdit
In plants of the Asteraceae, what appears to be a single "daisy"-type flower is actually a composite of several much smaller flowers, known as the capitulum or head. By visually presenting as a single flower, the capitulum functions in attracting pollinators, in the same manner that other "showy" flowering plants in numerous other, older, plant families have evolved to attract pollinators. The previous name for the family, Compositae, reflects the fact that what appears to be a single floral entity is in fact a composite of much smaller flowers.<ref name=CDF/>
The "petals" or "sunrays" in an "asteraceous" head are in fact individual strap-shaped<ref name=MDW/> flowers called ray flowers or ray florets, and the "sun disk" is made up of smaller, radially symmetric, individual flowers called disc flowers or disc florets. The word aster means "star" in Greek, referring to the appearance of most family members as a "celestial body with rays". The capitulum, which often appears to be a single flower, is often referred to as a head.<ref name=Spellenberg2001 /> In some species, the entire head is able to pivot its floral stem in the course of the day to track the sun (like a "smart" solar panel), thus maximizing the reflectivity of the entire floral unit and further attracting flying pollinators.<ref name=CDF/>
Nearest to the flower stem lie a series of small, usually green, scale-like bracts. These are known as phyllaries; collectively, they form the involucre, which serves to protect the immature head of florets during its development.Template:R The individual florets are arranged atop a dome-like structure called the receptacle.Template:R
The individual florets in a head consist, developmentally, of five fused petals (rarely four); instead of sepals, they have threadlike, hairy, or bristly structures,<ref name=Spellenberg2001 /> known collectively as a pappus, (plural pappi). The pappus surrounds the ovary and can, when mature and attached to a seed, adhere to animal fur or be carried by air currents, aiding in seed dispersal. The whitish, fluffy head of a dandelion, commonly blown on by children, consists of numerous seeds resting on the receptacle, each seed attached to its pappus. The pappi provide a parachute-like structure to help the seed travel from its point of origin to a more hospitable site.<ref name=CDF/>
1 – style with stigmas
2 – anthers
3 – corolla (petals); typically, in the ray flower, three petals are joined to form a strap (in other species, five petals can fuse to form a ligule)
4 – reduced calyx
4’ – Carduus acanthoides (left shaded circle): pappus: in many Asteraceae species, the calyx develops as a fibrous or bristly pappus
5 – inferior ovary: fused ovary consisting of two carpels, containing one abaxial ovule (basal placentation).
A ray flower is a two - or three-lobed, strap-shaped, individual flower, found in the head of most members of the Asteraceae.<ref name=CDF/><ref name=MDW/> The corolla of the ray flower may have two tiny, vestigial teeth, opposite to the three-lobed strap, or tongue, indicating its evolution by fusion from an ancestral, five-part corolla. In some species, the 3:2 arrangement is reversed, with two lobes, and zero or three tiny teeth visible opposite the tongue.
A ligulate flower is a five-lobed, strap-shaped, individual flower found in the heads of certain other asteraceous species.<ref name=CDF/> A ligule is the strap-shaped tongue of the corolla of either a ray flower or of a ligulate flower.Template:ClarifyTemplate:Example needed<ref name=MDW/> A disk flower (or disc flower) is a radially symmetric individual flower in the head, which is ringed by the ray flowers when both are present.<ref name=CDF/><ref name=MDW/> In some species, ray flowers may be arranged around the disc in irregular symmetry, or with a weakly bilaterally symmetric arrangement.<ref name=CDF/>
VariationsEdit
A radiate head has disc flowers surrounded by ray flowers. A ligulate head has all ligulate flowers and no disc flowers. When an Asteraceae flower head has only disc flowers that are either sterile, male, or bisexual (but not female and fertile), it is a discoid head.
Disciform heads possess only disc flowers in their heads, but may produce two different sex types (male or female) within their disciform head.
Some other species produce two different head types: staminate (all-male), or pistillate (all-female). In a few unusual species, the "head" will consist of one single disc flower; alternatively, a few species will produce both single-flowered female heads, along with multi-flowered male heads, in their "pollination strategy".<ref name=CDF/>
Floral structuresEdit
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The distinguishing characteristic of Asteraceae is their inflorescence, a type of specialised, composite flower head or pseudanthium, technically called a calathium or capitulum,<ref name=Beentje/><ref name=Usher/> that may look superficially like a single flower. The capitulum is a contracted raceme composed of numerous individual sessile flowers, called florets, all sharing the same receptacle.
A set of bracts forms an involucre surrounding the base 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 (dry and membranous) texture. The phyllaries 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 be subtended by a bract, called a "palea" or "receptacular bract". These bracts 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 Cichorioideae 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 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 four or five stamens.<ref name=Spellenberg2001 /> 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, and then, as the style elongates, is pushed out of the tube (nüdelspritze).
The pistil consists 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 seedsEdit
In members of the Asteraceae the fruit 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.<ref name=Spellenberg2001 /> 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.<ref name=McKenzieEtAl/> The mature seeds usually have little endosperm or none.<ref name=Judd/>
PollenEdit
The pollen of composites is typically echinolophate, a morphological term meaning "with elaborate systems of ridges and spines dispersed around and between the apertures."<ref name=Tomb1974/>
MetabolitesEdit
In Asteraceae, the energy store is generally in the form of inulin rather than starch. 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 that never contain iridoids.<ref name=Stevens/>
Asteraceae produce secondary metabolites, such as flavonoids and terpenoids. Some of these molecules can inhibit protozoan parasites such as Plasmodium, Trypanosoma, Leishmania and parasitic intestinal worms, and thus have potential in medicine.<ref name=PandaLuyten2018/>
TaxonomyEdit
HistoryEdit
Compositae, the original name for Asteraceae, were first described in 1740 by Dutch botanist Adriaan van Royen.Template:R Traditionally, two subfamilies were recognised: Asteroideae (or Tubuliflorae) and Cichorioideae (or Liguliflorae).Template:R The latter has been shown to be extensively paraphyletic, and has now been divided into 12 subfamilies, but the former still stands.<ref name=PaneroFunk/>Template:Update inline The study of this family is known as synantherology.
PhylogenyEdit
The phylogenetic tree of subfamilies presented below is based on Panero & Funk (2002)<ref name=PaneroFunk/> updated in 2014,<ref name=Deep/> and now also includes the monotypic Famatinanthoideae.<ref name=Deep/><ref name=FJNG/><ref name=FunkFragman-Sapir/>Template:Update inline The diamond (♦) denotes a very poorly supported node (<50% bootstrap support), the dot (•) a poorly supported node (<80%).<ref name=Stevens/>
The family includes over 32,000 currently accepted species, in over 1,900 genera (list) in 13 subfamilies.<ref name=PlantList/>Template:Update inline The number of species in the family Asteraceae is rivaled only by Orchidaceae.<ref name=Stevens/><ref name=PaneroCrozier/> Which is the larger family is unclear, because of the uncertainty about how many extant species each family includes.Template:Citation needed The four subfamilies Asteroideae, Cichorioideae, Carduoideae and Mutisioideae contain 99% of the species diversity of the whole family (approximately 70%, 14%, 11% and 3% respectively).Template:Citation needed
Because of the morphological complexity exhibited by this family, agreeing on generic circumscriptions has often been difficult for taxonomists. As a result, several of these genera have required multiple revisions.<ref name=Judd/>
Paleontology and evolutionary processesEdit
The oldest known fossils of members of Asteraceae are pollen grains from the Late Cretaceous of Antarctica, dated to ~76–66 mya (Campanian to Maastrichtian) and assigned to the extant genus Dasyphyllum. Barreda, et al. (2015) estimated that the crown group of Asteraceae evolved at least 85.9 mya (Late Cretaceous, Santonian) with a stem node age of 88–89 mya (Late Cretaceous, Coniacian).<ref name=PNAS2015/>
It is not known 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.<ref name=Stevens/> Heterocarpy, or the ability to produce different fruit morphs, has evolved and is common in Asteraceae. It allows seeds to be dispersed over varying distances and each is adapted to different environments, increasing chances of survival.<ref name=Gardocki/>
Etymology and pronunciationEdit
The original name Compositae is still valid under the International Code of Nomenclature for algae, fungi, and plants.<ref name=ICN185/> It refers to the "composite" nature of the capitula, which consist of a few or many individual flowers.Template:Citation needed
The alternative (as it came later) name Asteraceae (Template:IPAc-en) comes to international scientific vocabulary from Neo-Latin, from Aster, the type genus, + -aceae,<ref name=MWU/> a standardized suffix for plant family names in modern taxonomy. This genus name comes from the Classical Latin word Template:Wikt-lang, "star", which came from Ancient Greek {{#invoke:Lang|lang}} (Template:Transliteration), "star".<ref name=MWU/> It refers to the star-like form of the inflorescence.Template:Citation needed
The vernacular name daisy, widely applied to members of this family, is derived from the Old English name of the daisy (Bellis perennis): Template:Wikt-lang, meaning "day's eye". This is because the petals open at dawn and close at dusk.<ref name=daisy />
Distribution and habitatEdit
Asteraceae species have a widespread distribution, from subpolar to tropical regions in a wide variety of habitats. Most occur in hot desert and cold or hot semi-desert climates, and they are found on every continent but Antarctica. They are especially numerous in tropical and subtropical regions (notably Central America, eastern Brazil, the Mediterranean, the Levant, southern Africa, central Asia, and southwestern China).<ref name=PaneroCrozier/> The largest proportion of the species occur in the arid and semi-arid regions of subtropical and lower temperate latitudes.<ref name=BarkleyEtAl/> The Asteraceae family comprises 10% of all flowering plant species.<ref name=MandelEtAl2019/>
EcologyEdit
Template:More citations needed section Template:Multiple image Asteraceae are especially common in open and dry environments.<ref name=Judd/> Many members of Asteraceae are pollinated by insects, which explains their value in attracting beneficial insects, but anemophily 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. Anemochory (wind dispersal) is common, assisted by a hairy pappus. Epizoochory is another common method, in which the dispersal unit, a single cypsela (e.g. Bidens) or entire capitulum (e.g. Arctium) has hooks, spines or some structure to attach to the fur or plumage (or even clothes, as in the photo) of an animal just to fall off later far from its mother plant.
Some members of Asteraceae are economically important as weeds. Notable in the United States are Senecio jacobaea (ragwort),<ref name=WSNWCB/> Senecio vulgaris (groundsel),<ref name=WSNWCB2/> and Taraxacum (dandelion).<ref name=Gonzalez/> Some are invasive species in particular regions, often having been introduced by human agency. Examples include various tumbleweeds, Bidens, ragweeds, thistles, and dandelion.<ref name=MartinFores2018 /> Dandelion was introduced into North America by European settlers who used the young leaves as a salad green.<ref name=dandelion/> A number of species are toxic to grazing animals.<ref name=Spellenberg2001 />
UsesEdit
Asteraceae is an economically important family, providing products such as cooking oils, leaf vegetables like lettuce, sunflower seeds, artichokes, sweetening agents, coffee substitutes and herbal teas. Several genera are of horticultural importance, including pot marigold (Calendula officinalis), Echinacea (coneflowers), various daisies, fleabane, chrysanthemums, dahlias, zinnias, and heleniums. Asteraceae are important in herbal medicine, including Grindelia, yarrow, and many others.<ref name=Michel2020 />
Commercially important plants in 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).<ref name=ezcurra />
Plants are used as herbs and in herbal teas and other beverages. Chamomile, for example, comes from two different species: the annual Matricaria chamomilla (German chamomile) and the perennial Chamaemelum nobile (Roman chamomile). Calendula (known as pot marigold) is grown commercially for herbal teas and potpourri. Echinacea is used as a medicinal tea. The wormwood genus Artemisia includes absinthe (A. absinthium) and tarragon (A. dracunculus). Winter tarragon (Tagetes lucida), is commonly grown and used as a tarragon substitute in climates where tarragon will not survive.<ref name=jekkas />
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.<ref name=Aphididae/>
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Many species of this family possess medicinal properties and are used as traditional antiparasitic medicine.<ref name=PandaLuyten2018/>
Members of the family 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.<ref name=OdomJamesBerger2000/> Pollen from ragweed Ambrosia is among the main causes of so-called hay fever in the United States.<ref name=AsthmaAllergy/>
Asteraceae are also used for some industrial purposes. French Marigold (Tagetes patula) is common in commercial poultry feeds and its oil is extracted for uses in cola and the cigarette industry. The genera Chrysanthemum, Pulicaria, Tagetes, and Tanacetum contain species with useful insecticidal properties. Parthenium argentatum (guayule) is a source of hypoallergenic latex.<ref name=Aphididae/>
Several members of the family are copious nectar producers<ref name=Aphididae/> and are useful for evaluating pollinator populations during their bloom.Template:Citation needed 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 over winter.<ref name=SolidagoWinter/>
ReferencesEdit
See AlsoEdit
BibliographyEdit
External linksEdit
- Template:Commons category-inline
- Template:Wikispecies-inline
- Asteraceae at the Angiosperm Phylogeny Website
- Compositae.org – Compositae Working Group (CWG) and Global Compositae Database (GCD)
Template:Angiosperm families Template:Taxonbar Template:Authority control