Taraxacum

Taraxacum (/təˈræksəkəm/) is a genus of flowering plants in the family Asteraceae, which consists of species commonly known as dandelions. The scientific and hobby study of the genus is known as taraxacology. The genus has a near-cosmopolitan distribution, absent only from tropical and polar areas. Two of the most common species worldwide, T. officinale (the common dandelion) and T. erythrospermum (the red-seeded dandelion), are European species introduced into North America, where they are non-native. Dandelions thrive in temperate regions and can be found in yards, gardens, sides of roads, among crops, and in many other habitats.

Like other members of the family Asteraceae, they have very small flowers collected together into a composite flower head. Each single flower in a head is called a floret. In part due to their abundance, along with being a generalist species, dandelions are one of the most vital early spring nectar sources for a wide host of pollinators. Many Taraxacum species produce seeds asexually by apomixis, where the seeds are produced without pollination, resulting in offspring that are genetically identical to the parent plant.

The Latin name Taraxacum derives from the Arabic tarakhshaqūn, meaning "bitter herb". The Arabic term is possibly of Persian origin. Persian polymath Al-Razi used the word tarakhshaqūn in relation to dandelions, chicory, and endives. Al-Razi wrote "the tarakhshaqūn is like chicory, but more efficacious"; it is unclear exactly to which plant Al-Razi referred however. If Persian in origin, the word could have originally meant "bitter purslane" from تلک (talk, 'bitter') and چکش (chakūk, 'purslane'). Gerard of Cremona, in translating Arabic to Latin around 1170, spelled it tarasacon.

The English name, dandelion, is a borrowing of the French dent de lion meaning "lion's tooth", referring to the coarsely toothed leaves. The plant is also known as blowball, cankerwort, doon-head-clock, witch's gowan, milk witch, lion's-tooth, yellow-gowan, Irish daisy, monks-head, priest's-crown, and puff-ball; other common names include faceclock, pee-a-bed, wet-a-bed, swine's snout, white endive, and wild endive. The English folk name "piss-a-bed" (and the equivalent French pissenlit) refers to the strong diuretic effect of the plant's leaves. In Swedish, it is called maskroscode: swe promoted to code: sv (worm rose) after the thrip nymphs usually present on the plant.

Taraxacum species are tap-rooted, perennial, herbaceous plants, native to temperate areas of the Northern Hemisphere. The genus contains many species, which usually (or in the case of triploids, obligately) reproduce by apomixis, resulting in many local populations and endemism. In the British Isles alone, 234 microspecies (i.e. morphologically distinct clonal populations) are recognised in nine loosely defined sections. A number of Taraxacum species can act as ruderals, pioneer species that rapidly colonise disturbed soil. The common dandelion (T. officinale) has been introduced over much of the temperate world, and it is especially effective at spreading along roads, cemeteries, lawns, and pastures. A week or two after flowering, the dandelion's flower becomes a round seed head. The bracts, specialized leaves around the flower, curve backwards. The parachute ball fully opens into a sphere, and the yellow petals fall away. When development is complete, the mature seeds are attached to white, fluffy "parachutes", which easily detach from the seed head and glide on the wind, dispersing.

In general, the leaves are 5–25 centimetres (2.0–9.8 in) long or longer, simple, lobed-to-pinnatisect, forming a basal rosette above the central taproot. The flower heads range from yellow to orange, and they are open in the daytime, but closed at night. The heads are attached to a hollow stem (scape) that is usually leafless and rises 1–10 centimetres (0.4–3.9 in) or more above the ground. Stems and leaves exude a white, milky latex when broken. A rosette may produce several flowering stems at a time. The flower heads are 2–5 centimetres (0.8–2.0 in) in diameter and consist entirely of ray florets. The flower heads mature into spherical seed heads, sometimes called blowballs or clocks, containing many single-seeded fruits named cypselae, which are similar to achenes. Each cypsela is attached to a pappus of fine hair-like material, which enables anemochorous (wind-aided) dispersal over long distances.

The seeds are able to cover large distances when dispersed due to the unique morphology of the pappus, which works to create a unique type of vortex ring that stays attached to the seed rather than being sent downstream. In addition to the creation of this vortex ring, the pappus can adjust its morphology depending on the moisture in the air; this allows the plume of seeds to close up and reduce the chance to separate from the stem, waiting for optimal conditions that will maximise dispersal and germination.

In Taraxacum ovules, the megaspore mother cell is the only cell committed to enter the meiotic process. T. officinale is able to reproduce by a sexual meiotic process involving the production of haploid gametes, as well as by an asexual parthenogenetic meiotic process, referred to as apomixis, that produces diploid seeds genetically identical to the mother plant. Parthenogenesis permits the autonomous development of an unreduced egg cell into an embryo without the requirement of fertilisation.