Dandelion

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How to read a taxoboxDandelion
Macro dandelion Fcb981.JPG
Scientific classification
Kingdom: Plantae
Division: Magnoliophyta
Class: Magnoliopsida
Order: Asterales
Family: Asteraceae
Genus: Taraxacum
Cass.
Species

See text

Dandelion is the common name for any plants of the genus Taraxacum of the flowering plant family Asteraceae (known as the aster, daisy, or sunflower family). The dandelion, which is native to Europe and Asia but has spread to many other places, has a rosette of irregular, deeply notched leaves at the base of the plant, a hollow stem that exudes a milky sap, and flowers that are a composite flower head, consisting of many tiny flowers called florets (and composed only of ray florets, no disc florets). The term "dandelion" particularly is used with respect to the common species, Taraxacum officinale, whose solitary yellow flower head and "fruit" in the form of a ball-shaped cluster of wind-dispersed, one-seeded fruits make it a familiar sight—but which also is considered a weed in many places.

Taraxacum officinale has been well known for its nutritional and medicinal qualities for centuries, although the other species of dandelion are also are considered beneficial (Longe 2005). Dandelion is consumed as a leafy vegetable, whether cooked or raw (such as in salads or soup), and dandelion flower heads are used to make wine and jam. Medicinally, dandelion is used for a variety of purposes, including for treatment of liver disease, anemia, nervousness, and so forth. So useful is the plant medicinally, that the generic name is thought to be derived from the Greek taraxos, meaning "disorder," and akos, meaning remedy (Longe 2005). The specific designation officinale for the common variety indicates this herb was listed officially as a medicinal (Longe 2005). Beyond this, the dandelion plant, with its bright yellow flowers, can add beauty to a landscape.

Contents

Description

Asteraceae family

The family Asteraceae, to which Taraxacum belongs, is known as the aster, daisy, or sunflower family. It is a taxon of dicotyledonous flowering plants. The family name is derived from the genus Aster and refers to the star-shaped flower head of its members, epitomized well by the daisy. In addition to the dandelion and daisy, other well-known members of the family include lettuce, chicory, globe artichoke, safflower, chrysanthemums, ragwort, and sunflower.

Plants belonging to the Asteraceae share all the following characteristics (Judd et al. 1999). None of these traits, taken separately, can be considered synapomorphic.

  • The inflorescence is an involucrate capitulum (flower head)
  • Tubular/disc florets are actinomorphic, ligulate/ray florets are zygomorphic
  • Anthers are syngenesious, i.e. with the stamens fused together at their edges, forming a tube
  • The ovary has basal arrangement of the ovules
  • One ovule per ovary
  • The calyx (sepals) of the florets are modified to form a pappus, a tuft of hairs, which often appears on the mature fruit
  • The fruit is an achene
  • In the essential oils Sesquiterpenes are present, but iridoids are lacking.

The most common characteristic of all these plants is an inflorescence or flower head (formerly composite flower): a densely packed cluster of numerous, small, individual flowers, usually called florets (meaning "small flowers"). Plants in the family Asteraceae typically have one or both of two kinds of florets. The outer perimeter of a flower head like that of a sunflower is composed of florets possessing a long strap-like petal, termed a ligule; these are the ray florets. The inner portion of the flower head (or disc) is composed of small flowers with tubular corollas; these are the disc florets.

Dandelion (Taraxacum genus)

A flowering dandelion.

The genus Taraxacum comprises 60 or more species, depending of the taxonomic scheme. The genus is taxonomically very complex, with numerous macrospecies, and polyploidy is also common. Over 250 species have been recorded in the British Isles alone (Richards 1972). However, some botanists take a much narrower viewpoint, and only accept a total of about 60 species.

Dandelions are tap-rooted biennial or perennial herbaceous plants, native to temperate areas of the Northern Hemisphere of the Old World. The leaves are 5-25 cm long, simple and basal, entire or lobed, forming a rosette above the central taproot. As the leaves grow outward, they push down the surrounding vegetation, such as grass in a lawn, killing the vegetation by cutting off the sunlight.

Macro photo of dandelion seed dispersal.

A bright yellow flower head (which is open in the daytime but closes at night) is borne singly on a hollow stem (scape), which rises 4-30 cm above the leaves and exudes a milky sap (latex) when broken. A rosette may produce several flowering stems at a time. The flower head is 2-5 cm in diameter and is limited entirely to ray florets, no disc florets.

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 down to allow the seeds to disperse; the outer bracts are always reflexed downward. Some species drop the "parachute" (called a pappus, modified sepals) from the achenes. Between the pappus and the achene, there is a stalk called a beak, which elongates as the fruit matures. The beak breaks off from the achene quite easily.

A dandelion clock.

The flower matures into a globe of fine filaments that are usually distributed by wind, carrying away the seed-containing achenes. This globe (receptacle) is called the "clock."

A microscopic view of a pappus from a dandelion clock.

After pollination, the dandelion flower dries out for about 10 days and then the seed-bearing parachutes expand and lift out of the dried flower head. The dried part of the flower drops off and the parachute ball opens into a full sphere. The parachute drops off when the seed strikes an obstacle. Often dandelions can be observed growing in a crevice near a wall, because the blowing fruits hit the wall and the feathery pappi drop off, sending the dandelion seeds to the base of the obstacle where they germinate. After the seed is released, the parachutes lose their feathered structure and take on a fuzzy, cotton-like appearance, often called "dandelion snow."

In northern areas and places where the dandelion is not native, it has become a weedy species, exploiting disturbed ground in human environments. They have become established in the Americas, Australia, and New Zealand as weeds. They are now common plants throughout all temperate regions.

Taraxacum species reproduce asexually by means of apomixis and seed production commonly occurs without pollination (Doll and Trower 2002).

Dandelions are used as food plants by the larvae of some species of Lepidoptera.

False dandelions

Dandelions are so similar to catsears (Hypochoeris) that catsears are also known as "false dandelions." Both plants carry similar flowers which form into windborne seeds. However, catsear's flowering stems are forked and solid, whereas dandelions possess unforked stems that are hollow. Both plants have a rosette of leaves and a central taproot. However, the leaves of dandelions are jagged in appearance, whereas those of catsear are more lobe-shaped and hairy. Other plants with similar flowers include hawkweeds (Hieracium) and hawksbeards (Crepis). These are both readily distinguished by their branched flowering stems.

Etymology

Loewenzahn bluete unten.jpg

The name dandelion is derived from the French, dent-de-lion, which is literally "lion's tooth," referring to the sharply-lobed leaves of the plant. The English spelling reflects the French pronunciation at the time this French word was absorbed into English (CBC 2007). The first written usage of the word occurs in a herbal dated 1373, but there is a 1363 document in which the word "dandelion" was used as a proper name (Willelmus Dawndelyon).

Selected species

  • Taraxacum officinale (syn. T. officinale subsp. vulgare), Common Dandelion. Found in many forms, but differs at least from the following species:
  • Taraxacum albidum, a white-flowering Japanese dandelion.
  • Taraxacum japonicum, Japanese dandelion. No ring of smallish, downward-turned leaves under the flowerhead.
  • Taraxacum laevigatum (syn. T. erythrospermum), Red-seeded Dandelion; achenes reddish brown and leaves deeply cut throughout length. Inner bracts' tips are hooded.

Seed development and genetics

A microscopic view of a dandelion clock showing the pericarp and the achenes.

As previously mentioned, the taxonomical situation of the genus is quite complex, mainly because many dandelions are genetically triploid. An odd number of chromosomes usually is associated with sterility, but dandelions with this karyotype can reproduce without fertilization, by a process called apomixis (Kells et al.) In these individuals, flowers are largely useless vestigial structures, although they may still produce a small percentage of fertile pollen, keeping some genetic contact with sexual individuals. Diploid dandelions develop seeds after cross-pollination and are outcrossing, or self-incompatible. In most zones of southern Europe and Asia, dandelion populations are sexual or mixed sexual-apomictic, while in northern countries only triploid and tetraploid apomicts are present, as is in the zones where it is not native. This seems to be linked to higher temperatures, survival of pre-glacial populations, and human impact, but the subject is still being studied.

There are usually 54 to 172 seeds produced per head, but a single plant can produce more than 2000 seeds a year. It has been estimated that more than 97,000,000 seeds/hectare could be produced every year by a dense stand of dandelions.

Uses

The dandelion's taproot, on left in this drawing, makes this plant very difficult to uproot; the top of the plant breaks away, but the root stays in the ground and can sprout again.

While the dandelion is considered a weed by many gardeners and lawn owners, and difficult to eliminate with its long taproot, the plant has also been prized over the centuries for several culinary and medicinal uses. Dandelions are grown commercially on a small scale as a leaf vegetable. The plant can be eaten cooked or raw in various forms, such as in soup or salad. They are probably closest in character to mustard greens. Usually the young leaves and unopened buds are eaten raw in salads, while older leaves are cooked. Raw leaves have a slightly bitter taste. Dandelion salad is often accompanied with hard boiled eggs.

Dandelion flowers can be used to make dandelion wine. The recipe usually contains citrus fruit. Another recipe using the plant is dandelion flower jam. Ground roasted dandelion root can be used as a coffee substitute. Drunk before meals, it is believed to stimulate digestive functions. Sold in most health food stores, often in a mixture, it is considered an excellent cleansing tonic for the liver.

Dandelion is nutritious. It is high in potassium, calcium, and lecithin, as well as providing iron, magnesium, proteins, zinc, and several B vitamins and vitamins C and E (Longe 2005). The leaf includes glycosides, carotenoids, terpenoids, choline, potassium salts and other minerals, and the roots also have glycosides, tannins, sterols, triterpenes, choline, asparagin, and inulin (Longe 2005).

Medicinally, the entire dandelion plant is used as a general tonic (particularly a liver tonic), or it may be taken as an infusion of the leaf, a root docotion, a juice extraction, or a tincture (solution) (Longe 2005). Dandelion root is a registered drug in Canada, sold as a diuretic. Dandelions are so potent in this effect, that children have been known to wet the bed the night after skin contact from playing with them (Saccoccio 2007). A leaf decoction can be drunk to "purify the blood," for the treatment of anemia, jaundice, and also for nervousness. The milky latex has been used as a mosquito repellent; the milk is also applied to warts, helping get rid of them without damaging the surrounding skin. A dye can also be obtained from the roots of the plant. A new mixture of roasted roots is sold as a product called DandyBlend which tastes like coffee after the inulin in the dandelion is roasted.

Herbalists consider the dandelion to be effective in the treatment of liver diseases, even in extreme cases such as cirrhosis (Longe 2005). It is considered to increase bile production and cleanse the bloodstream, and useful for gall bladder problems and other internal organs, such as pancreas, kidneys, stomach, and spleen (Longe 2005). Dandelion contains luteolin, an antioxidant, and has demonstrated antioxidant properties without cytotoxicity (Hu and Kitts 2004).

"Dandelion and Burdock" is a soft drink that has long been popular in the United Kingdom, with authentic recipes sold by health food shops. It is unclear whether cheaper supermarket versions actually contain either plant.

This plant also is useful in farming, because its deep, strong roots break up hardpan.

Caffeic acid and carcinogenicity

Caffeic acid is a secondary plant metabolite produced in dandelion, yarrow, horsetail, and whitethorn. Despite its name, it is totally unrelated to caffeine. Recent studies have revealed this acid may be carcinogenic. Caffeic acid was tested for carcinogenicity by oral administration in mice, it produced renal cell adenomas in females, and a high incidence of renal tubular cell hyperplasia in animals of each sex (Natarajan et al. 1996). However, other research shows that bacteria present in the rats' guts may alter the formation of metabolites of caffeic acid (Peppercorn and Goldman 1972; Gonthier 2003). Also, there have been no known ill-effects of caffeic acid in humans.


References

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