Asteraceae or Compositae (commonly referred to as the aster, daisy, or sunflower family), are an exceedingly large and widespread family of Angiospermae.  The group has more than 23,000 currently accepted species, spread across 1620 genera and 12 subfamilies. In terms of numbers of species, Asteraceae is rivaled only by Orchidaceae. (Which of the two families is actually larger is unclear, owing to uncertainty about exactly how many species exist in each family). The main feature of the family is the composite flower type in the form of capitula surrounded by involucral bracts.The name "Asteraceae" comes from Aster, the most prominent generum in the family, that derives from the Greek ?st?? meaning star, and is connected with its inflorescence star form. As for the term "Compositae", more ancient but still valid, it obviously makes reference to the fact that the family is one of the few angiosperms that have composite flowers. This family has a remarkable ecological and economical importance, and is present from the polar regions to the tropics, colonizing all available habitats. The Asteraceae may represent as much as 10% of autochthon flora in many regions of the world.
Most members of Asteraceae are herbaceous, but a significant number are also shrubs, vines and trees. The family has a worldwide distribution, and is most common in the arid and semi-arid regions of subtropical and lower temperate latitudes.
Asteraceae is an economically important family. Some members provide products including cooking oils, lettuce, sunflower seeds, artichokes, sweetening agents, coffee substitutes and teas. Several genera are popular with the horticultural community, including marigold, pot marigold (also known as calendula), cone flowers, various daisies, fleabane, chrysanthemums, dahlias, zinnias, and heleniums. Asteraceae are important in herbal medicine, including Grindelia, echinacea, yarrow and many others. A number of species have come to be considered invasive, including, most notably in North America, dandelion, which was originally introduced by European settlers who used the young leaves as a salad green.
The Latin name 'Asteraceae' is derived from the type genus Aster, which is a Greek term that means "star". '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.
The vernacular name daisy, widely applied to members of this family, is derived from its Old English meaning, dægesege, from dæges eage meaning "day's eye," and this was because the petals (of Bellis perennis) open at dawn and close at dusk.
Asteraceae have a cosmopolitan distribution, and are found everywhere except Antarctica and the extreme Arctic. 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).
Asteraceae are mostly herbaceous plants, but some shrubs, trees and climbers do exist. Asteraceae are generally easy to distinguish from other plants, mainly because of their characteristic inflorescence and other shared characteristics. However, determining genera and species is notoriously difficult (see "damned yellow composite" for example).
Roots and stems
Asteraceae generally produce taproots, but sometimes they possess fibrous root systems. Stems are generally erect, but can be 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.
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 lobed or toothed.
Flower diagram of Carduus (Carduoideae) shows (outermost to innermost): subtending bract and stem axis; fused calyx; fused corolla; stamens fused to corolla; gynoecium with two carpels and one locule
A typical Asteraceae flower head (here Bidens torta) showing the individual flowers
Ray floret (as in Cichorioideae, and the outer florets in Asteroideae)
Disc floret (as in Asteroideae)
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 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.
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 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 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, 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 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. The mature seeds usually have little endosperm or none.
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.
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.
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.
Seeds are dispersed by the wind in Carlina
Epizoochory in Bidens tripartita
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.
A Dahlia cultivar
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).
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.
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).
Compositae have also been used for industrial purposes. Common in all commercial poultry feed, marigold (Tagetes patula) is grown primarily in Mexico and central American nations. Marigold oil, extracted from Tagetes minuta, is used in the cola and cigarette industries.
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 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 over winter.
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.