Asteraceae
Asteraceae 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. 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 of Antarctica, dated to million years ago. It is estimated that the crown group of Asteraceae evolved at least 85.9 mya with a stem node age of 88–89 mya.
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.
Description
Members of the Asteraceae are mostly herbaceous plants, but some shrubs, vines, and trees 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. Nonetheless, determining genera and species of some groups, such as Hieracium, is notoriously difficult.Roots
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.Stems
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, or latex, which is particularly common among the Cichorioideae.Leaves
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 or latex can also be present in the leaves.Inflorescences
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 flowers in the capitulum may resemble petals, and the involucral bracts may look like a calyx. Notable exceptions include Hecastocleis shockleyi and the species of the genus Corymbium, which have one-flowered bisexual capitulas, Gundelia with one-flowered unisexual capitulas, and Gymnarrhena micrantha with one-flowered female capitulas and few flowered male capitulas.Floral heads
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 attracts 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.The "petals" or "sunrays" in an "asteraceous" head are in fact individual strap-shaped 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. In some species, the entire head is able to pivot its floral stem in the course of the day to track the sun, thus maximizing the reflectivity of the entire floral unit and further attracting flying pollinators.
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. The individual florets are arranged atop a dome-like structure called the receptacle.
The individual florets in a head consist, developmentally, of five fused petals ; instead of sepals, they have threadlike, hairy, or bristly structures, known collectively as a pappus. 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.
File:Leucanthemum diagram.svg|thumb|upright=3|center|alt=refer to caption|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 ; typically, in the ray flower, three petals are joined to form a strap
4 – reduced calyx
4' – Carduus acanthoides : 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.
A ray flower is a two - or three-lobed, strap-shaped, individual flower, found in the head of most members of the Asteraceae. 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. A ligule is the strap-shaped tongue of the corolla of either a ray flower or of a ligulate flower. A disk flower is a radially symmetric individual flower in the head, which is ringed by the ray flowers when both are present. In some species, ray flowers may be arranged around the disc in irregular symmetry, or with a weakly bilaterally symmetric arrangement.
Variations
When an Asteraceae flower head has only disc flowers that are bisexual, it is a discoid head. A subtype of discoid heads are radiant heads, which have larger, dilated, and often bilateral outer florets.Disciform heads include disk florets, which may be male or bisexual, and surrounding florets that may be naked or tubular and female.
A radiate head has disc florets surrounded by an outer portion of ray florets known as a lamina.
A ligulate head has only ligulate florets.
Some other species produce two different head types: staminate, or pistillate. 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".
Floral structures
The distinguishing characteristic of Asteraceae is their inflorescence, a type of specialised, composite flower head or pseudanthium, technically called a calathium or capitulum, 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 or have a scarious texture. The phyllaries can be free or fused, and arranged in one to many rows, overlapping like the tiles of a roof or not.
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.
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. The filaments are fused to the corolla, while the anthers are generally connate, thus forming a sort of tube around the style. 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.
File:ZinniaFloret 10 prob4 62.webm|thumb|thumbtime=71|Time-lapse of Zinnia disc floret petals opening, their styles providing secondary pollen presentation. Visiting insects shown at normal speed. A thrips and two hoverflies eat the piled pollen. Two skipper butterflies reach the deep nectaries.
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.