Asteraceae

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.[5]

Stems are herbaceous, aerial, branched, and cylindrical with glandular hairs, generally erect, but can be prostrate to ascending. The stems can contain secretory canals with resin,[5] or latex, which is particularly common among the Cichorioideae.[8]

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[5] or latex[8] also can be present in the leaves.

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.

A typical Asteraceae flower head showing the (five) individual ray florets and the (approximately 16) disk florets of a specimen of (Bidens torta)

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.[9]

The "petals" or "sunrays" in an "asteraceous" head are in fact individual strap-shaped[10] 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.[11] 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.[9]

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.[9]^: 29 The individual florets are arranged atop a dome-like structure called the receptacle.[9]

The individual florets in a head consist, developmentally, of five fused petals (rarely four); instead of sepals, they have threadlike, hairy, or bristly structures,[11] 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.[9]

Schemes and floral diagrams of the different floret types of the Asteraceae: Leucanthemum vulgare: a = disc flower; b = ray flower.
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.[9][10] 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.[9] A ligule is the strap-shaped tongue of the corolla of either a ray flower or of a ligulate flower.[10] 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.[9][10] In some species, ray flowers may be arranged around the disc in irregular symmetry, or with a weakly bilaterally symmetric arrangement.[9]

Flower diagram of Carduus (Carduoideae) shows (outermost to innermost): subtending bract and stem axis; calyx forming a pappus; fused corolla; stamens fused to corolla; gynoecium with two carpels and one locule

Discoid flowerheads of Delairea odorata.

The distinguishing characteristic of Asteraceae is their inflorescence, a type of specialised, composite flower head or pseudanthium, technically called a calathium or capitulum,[12][13] 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.[11] 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.

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.[11] 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.[14] The mature seeds usually have little endosperm or none.[6]

The pollen of composites is typically echinolophate, a morphological term meaning "with elaborate systems of ridges and spines dispersed around and between the apertures."[15]

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.[16]

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.[17]

Compositae, the original name for Asteraceae, were first described in 1740 by Dutch botanist Adriaan van Royen.[18]^{: 117–118} Traditionally, two subfamilies were recognised: Asteroideae (or Tubuliflorae) and Cichorioideae (or Liguliflorae).[19]^: 242 The latter has been shown to be extensively paraphyletic, and has now been divided into 12 subfamilies, but the former still stands.[20] The study of this family is known as synantherology.

The phylogenetic tree of subfamilies presented below is based on Panero & Funk (2002)[20] updated in 2014,[21] and now also includes the monotypic Famatinanthoideae.[21][22][23] The diamond (♦) denotes a very poorly supported node (<50% bootstrap support), the dot (•) a poorly supported node (<80%).[16]

The family includes over 32,000 currently accepted species, in over 1,900 genera (list) in 13 subfamilies.[3] The number of species in the family Asteraceae is rivaled only by Orchidaceae.[16][24] Which is the larger family is unclear, because of the uncertainty about how many extant species each family includes. 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).

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.[6]

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).[1]

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.[16] 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.[25]

The original name Compositae is still valid under the International Code of Nomenclature for algae, fungi, and plants.[26] It refers to the "composite" nature of the capitula, which consist of a few or many individual flowers.

The alternative (as it came later) name Asteraceae (English: /ˌæstəˈreɪsi, -siˌaɪ, -siˌeɪ, -siˌiː/) comes to international scientific vocabulary from Neo-Latin, from Aster, the type genus, + -aceae,[27] a standardized suffix for plant family names in modern taxonomy. This genus name comes from the Classical Latin word aster, "star", which came from Ancient Greek ἀστήρ (astḗr), "star".[27] It refers to the star-like form of the inflorescence.

The vernacular name daisy, widely applied to members of this family, is derived from the Old English name of the daisy (Bellis perennis): dæġes ēaġe, meaning "day's eye". This is because the petals open at dawn and close at dusk.[28]