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Taraxacum sect. Ruderalia
Taraxacum (/təˈræksəkʊm/) is a large genus of flowering plants in the family Asteraceae, which consists of species commonly known as dandelions. The genus is native to Eurasia and North America, but the two commonplace species worldwide, T. officinale and T. erythrospermum, were introduced from Europe and now propagate as wildflowers. Both species are edible in their entirety. The common name dandelion (/ˈdændɪlaɪ.ən/ DAN-di-ly-ən, from French dent-de-lion, meaning "lion's tooth") is given to members of the genus. 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.
Description & appearance
The species of Taraxacum 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 are recognised in nine loosely defined sections, of which 40 are "probably endemic".
In general, the leaves are 5–25 cm (2" to 10") long or longer, simple, lobed, and form a basal rosette above the central taproot. The flower heads are yellow to orange coloured, and are open in the daytime, but closed at night. The heads are borne singly on a hollow stem (scape) that is usually leafless and rises 1–10 cm (½" to 4") or more above the leaves. 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 cm (1" to 2") in diameter and consist entirely of ray florets. The flower heads mature into spherical seed heads sometimes called blowballs or clocks (in both British and American English) containing many single-seeded fruits called achenes. Each achene is attached to a pappus of fine hair-like material which enables wind-aided dispersal over long distances.
The flower head is surrounded by bracts (sometimes mistakenly called sepals) in two series. The inner bracts are erect until the seeds mature, then flex downward to allow the seeds to disperse. The outer bracts are often reflexed downward, but remain appressed in plants of the sections Palustria and Spectabilia. Some species drop the "parachute" from the achenes; the hair-like parachutes are called pappus, and they are modified sepals. Between the pappus and the achene is a stalk called a beak, which elongates as the fruit matures. The beak breaks off from the achene quite easily, separating the seed from the parachute.
A number of species of Taraxacum are seed-dispersed ruderals that rapidly colonize disturbed soil, especially the common dandelion (T. officinale), which has been introduced over much of the temperate world. After flowering is finished, the dandelion flower head dries out for a day or two. The dried petals and stamens drop off, the bracts reflex (curve backwards), and the parachute ball opens into a full sphere. When development is complete, the mature seeds are attached to white, fluffy "parachutes" which easily detach from the seedhead and glide by wind, dispersing.
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 maximize dispersal and germination.
Dandelions are thought to have evolved about 30 million years ago in Eurasia. Fossil seeds of Taraxacum tanaiticum have been recorded from the Pliocene of southern Russia. Dandelions have been used by humans for food and as an herb for much of recorded history. They were well known to ancient Egyptians, Greeks and Romans, and are recorded to have been used in traditional Chinese medicine for over a thousand years. The plant was used as food and medicine by Native Americans. Dandelions probably arrived in North America on the Mayflower—not as stowaways, but brought on purpose for their medicinal benefits.
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