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Ginkgo (Ginkgo biloba; in Chinese and Japanese 銀杏, pinyin romanization: yín xìng, Hepburn romanization: ichō or ginnan), also known as the Maidenhair Tree after Adiantum, is a unique species of tree with no close living relatives. The ginkgo is classified in its own division, the Ginkgophyta, comprising the single class Ginkgoopsida, order Ginkgoales, family Ginkgoaceae, genus Ginkgo and is the only extant species within this group. It is one of the best-known examples of a living fossil, because Ginkgoales other than G. biloba are not known from the fossil record after the Pliocene.

For centuries it was thought to be extinct in the wild, but is now known to grow in at least two small areas in Zhejiangmarker province in Eastern Chinamarker, in the Tian Mu Shan Reserve. However, recent studies indicate high genetic uniformity among ginkgo trees from these areas, arguing against a natural origin of these populations and suggesting that the ginkgo trees in these areas may have been planted and preserved by Chinese monks over a period of about 1000 years. Whether native ginkgo populations still exist has not been demonstrated unequivocally.

The relationship of Ginkgo to other plant groups remains uncertain. It has been placed loosely in the divisions Spermatophyta and Pinophyta, but no consensus has been reached. Since Ginkgo seeds are not protected by an ovary wall, it can morphologically be considered a gymnosperm. The apricot-like structures produced by female ginkgo trees are technically not fruits, but are seeds that have a shell that consists of a soft and fleshy section (the sarcotesta), and a hard section (the sclerotesta).

Description



Ginkgos are very large trees, normally reaching a height of 20–35 m (66–115 feet), with some specimens in China being over 50 m (164 feet). The tree has an angular crown and long, somewhat erratic branches, and is usually deep rooted and resistant to wind and snow damage. Young trees are often tall and slender, and sparsely branched; the crown becomes broader as the tree ages. During autumn, the leaves turn a bright yellow, then fall, sometimes within a short space of time (1–15 days). A combination of resistance to disease, insect-resistant wood and the ability to form aerial roots and sprouts makes ginkgos long-lived, with some specimens claimed to be more than 2,500 years old.

Ginkgo is a relatively shade-intolerant species that (at least in cultivation) grows best in environments that are well-watered and well-drained. The species shows a preference for disturbed sites; in the "semi-wild" stands at Tian Mu Shan, many specimens are found along stream banks, rocky slopes, and cliff edges. Accordingly, Ginkgo retains a prodigious capacity for vegetative growth. It is capable of sprouting from embedded buds near the base of the trunk (lignotubers, or basal chi chi) in response to disturbances, such as soil erosion. Old individuals are also capable of producing aerial roots (chi chi) on the undersides of large branches in response to disturbances such as crown damage; these roots can lead to successful clonal reproduction upon contacting the soil. These strategies are evidently important in the persistence of Ginkgo; in a survey of the "semi-wild" stands remaining in Tian Mu Shan, 40% of the Ginkgo specimens surveyed were multi-stemmed, and few saplings were present.
Trunk bark


Stem

Ginkgo branches grow in length by growth of shoots with regularly spaced leaves, as seen on most trees. From the axils of these leaves, "spur shoots" (also known as short shoots) develop on second-year growth. Short shoots have very short internode (so they may grow only one or two centimeters in several years) and their leaves are usually unlobed. They are short and knobby, and are arranged regularly on the branches except on first-year growth. Because of the short internodes, leaves appear to be clustered at the tips of short shoots, and reproductive structures are formed only on them (see pictures below - seeds and leaves are visible on short shoots). In Ginkgos, as in other plants that possess them, short shoots allow the formation of new leaves in the older parts of the crown. After a number of years, a short shoot may change into a long (ordinary) shoot, or vice versa.

Leaves

The leaves are unique among seed plants, being fan-shaped with veins radiating out into the leaf blade, sometimes bifurcating (splitting) but never anastomosing to form a network. Two veins enter the leaf blade at the base and fork repeatedly in two; this is known as dichotomous venation. The leaves are usually 5–10 cm (2-4 inches), but sometimes up to 15 cm (6 inches) long. The old popular name "Maidenhair tree" is because the leaves resemble some of the pinnae of the Maidenhair fern Adiantum capillus-veneris.

Leaves of long shoots are usually notched or lobed, but only from the outer surface, between the veins. They are borne both on the more rapidly-growing branch tips, where they are alternate and spaced out, and also on the short, stubby spur shoots, where they are clustered at the tips.

Reproduction

Ginkgos are dioecious, with separate sexes, some trees being female and others being male. Male plants produce small pollen cones with sporophylls each bearing two microsporangia spirally arranged around a central axis.

Female plants do not produce cones. Two ovules are formed at the end of a stalk, and after pollination, one or both develop into seeds. The seed is 1.5–2 cm long. Its fleshy outer layer (the sarcotesta) is light yellow-brown, soft, and fruit-like.It is attractive in appearance, but contains butanoic acid (also known as butyric acid) and smells like rancid butter (which contains the same chemical) or feces when fallen. Beneath the sarcotesta is the hard sclerotesta (what is normally known as the "shell" of the seed) and a papery endotesta, with the nucellus surrounding the female gametophyte at the center.

The fertilization of ginkgo seeds occurs via motile sperm, as in cycads, ferns, mosses and algae. The sperm are large (about 250-300 micrometres) and are similar to the sperm of cycads, which are slightly larger. Ginkgo sperm were first discovered by the Japanese botanist Sakugoro Hirase in 1896. The sperm have a complex multi-layered structure, which is a continuous belt of basal bodies that form the base of several thousand flagella which actually have a cilia-like motion. The flagella/cilia apparatus pulls the body of the sperm forwards. The sperm have only a tiny distance to travel to the archegonia, of which there are usually two or three. Two sperm are produced, one of which successfully fertilizes the ovule. Although it is widely held that fertilization of ginkgo seeds occurs just before or after they fall in early autumn, embryos ordinarily occur in seeds just before and after they drop from the tree.

Image:Ginkgo-biloba-male.JPG|Ginkgo pollen-bearing conesImage:GingkoFruitingTwigSpring.jpg|Ovules ready for fertilizationImage:Ginkgo embryo and gametophyte.jpg|Female gametophyte, dissected from a seed freshly shed from the tree, containing a well-developed embryoImage:Ginkgo biloba0.jpg|Immature ginkgo ovules and leavesImage:Gingko fg01.jpg|Autumn leaves and fallen seedsImage:GinkgoSaplings.jpg|A forest of saplings sprout among last year's seeds

Distribution and habitat

Although Ginkgo biloba and other species of the genus were once widespread throughout the world, the tree currently occurs in the wild only in the northwest of Zhejiang provincemarker in the Tianmu Shan mountain reserve in eastern Chinamarker, but even its status as a naturally occurring species there is questionable. In other areas of China it has been long cultivated and it is common in the southern third of the country. It has also been commonly cultivated in North America for over 200 years, but during that time it has never become significantly naturalised.

Where it occurs in the wild it is found infrequently in deciduous forests and valleys on acidic loess (i.e. fine, silty soil) with good drainage. The soil it inhabits is typically in the pH range of 5 to 5.5.

Taxonomy and naming

The species was initially described by the father of taxonomy Linnaeus in 1771, the specific epithet biloba derived from the Latin bis 'two' and loba 'lobed', referring to the shape of the leaves.

Etymology

The older Chinese name for this plant is 銀果 yínguǒ ('silver fruit').The most usual names today are 白果 bái guǒ ('white fruit') and yínxìng ('silver apricot'). The former name was borrowed directly in Vietnamese (as bạch quả).The latter name was borrowed in Japanese (as ぎんなん "ginnan") and Korean (as 은행 "eunhaeng"), when the tree itself was introduced from China.

The scientific name Ginkgo appears to be due to a process akin to folk etymology.Chinese characters typically have multiple pronunciations in Japanese, and the characters 銀杏 used for ginnan can also be pronounced ginkyō.Engelbert Kaempfer, the first Westerner to see the species in 1690, wrote down this pronunciation in his Amoenitates Exoticae (1712); his y was misread as a g, and the misspelling stuck.

Palaeontology

The Ginkgo is a living fossil, with fossils recognisably related to modern Ginkgo from the Permian, dating back 270 million years. The most plausible ancestral group for the order Ginkgoales is the Pteridospermatophyta, also known as the "seed ferns," specifically the order Peltaspermales. The closest living relatives of the clade are the cycads, which share with the extant G. biloba the characteristic of motile sperm. Fossils attributable to the genus Ginkgo first appeared in the Early Jurassic, and the genus diversified and spread throughout Laurasia during the middle Jurassic and Early Cretaceous. It declined in diversity as the Cretaceous progressed, and by the Paleocene, Ginkgo adiantoides was the only Ginkgo species left in the Northern Hemispheremarker while a markedly different (and poorly documented) form persisted in the Southern Hemispheremarker. At the end of the Pliocene, Ginkgo fossils disappeared from the fossil record everywhere except in a small area of central China where the modern species survived. It is doubtful whether the Northern Hemisphere fossil species of Ginkgo can be reliably distinguished. Given the slow pace of evolution and morphological similarity between members of the genus, there may have been only one or two species existing in the Northern Hemisphere through the entirety of the Cenozoic: present-day G. biloba (including G. adiantoides) and G. gardneri from the Paleocene of Scotlandmarker.

At least morphologically, G. gardneri and the Southern Hemisphere species are the only known post-Jurassic taxa that can be unequivocally recognised. The remainder may have been ecotypes or subspecies. The implications would be that G. biloba had occurred over an extremely wide range, had remarkable genetic flexibility and, though evolving genetically, never showed much speciation. While it may seem improbable that a species may exist as a contiguous entity for many millions of years, many of the Ginkgo's life-history parameters fit. These are: extreme longevity; slow reproduction rate; (in Cenozoic and later times) a wide, apparently contiguous, but steadily contracting distribution coupled with, as far as can be demonstrated from the fossil record, extreme ecological conservatism (restriction to disturbed streamside environments).

Image:Ginkgo biloba MacAbee BC.jpg|Ginkgo biloba Eocene leaf from the MacAbee, B.C.marker, Canada.Image:Fossil Plant Ginkgo.jpg|Fossil Ginkgo leaves from the Jurassic of England

Modern-day G. biloba grows best in environments that are well-watered and drained, and the extremely similar fossil Ginkgo favored similar environments: the sediment record at the majority of fossil Ginkgo localities indicates it grew primarily in disturbed environments along streams and levees. Ginkgo therefore presents an "ecological paradox" because while it possesses some favorable traits for living in disturbed environments (clonal reproduction) many of its other life-history traits (slow growth, large seed size, late reproductive maturity) are the opposite of those exhibited by modern plants that thrive in disturbed settings.

Given the slow rate of evolution of the genus, it is possible that Ginkgo represents a pre-angiosperm strategy for survival in disturbed streamside environments. Ginkgo evolved in an era before flowering plants, when ferns, cycads, and cycadeoids dominated disturbed streamside environments, forming a low, open, shrubby canopy. Ginkgo's large seeds and habit of "bolting" - growing to a height of 10 m before elongating its side branches - may be adaptions to such an environment. The fact that diversity in the genus Ginkgo drops through the Cretaceous (along with that of ferns, cycads, and cycadeoids) at the same time that flowering plants were on the rise, supports the notion that flowering plants with better adaptations to disturbance displaced Ginkgo and its associates over time.

Ginkgo has been used for classifying plants with leaves that have more than four veins per segment, while Baiera for those with fewer than four veins per segment. Sphenobaiera has been used to classify plants with a broadly wedge-shaped leaf that lacks a distinct leaf stem. Trichopitys is distinguished by having multiple-forked leaves with cylindrical (not flattened) thread-like ultimate divisions; it is one of the earliest fossils ascribed to the Ginkgophyta.

Cultivation and uses

Ginkgo has long been cultivated in Chinamarker; some planted trees at temples are believed to be over 1,500 years old. The first record of Europeans encountering it is in 1690 in Japanese temple gardens, where the tree was seen by the German botanist Engelbert Kaempfer. Because of its status in Buddhism and Confucianism, the Ginkgo is also widely planted in Korea and parts of Japan; in both areas, some naturalization has occurred, with Ginkgos seeding into natural forests.

In some areas, most intentionally planted Ginkgos are male cultivars grafted onto plants propagated from seed, because the male trees will not produce the malodorous seeds. The popular cultivar 'Autumn Gold' is a clone of a male plant.

Ginkgos adapt well to the urban environment, tolerating pollution and confined soil spaces. They rarely suffer disease problems, even in urban conditions, and are attacked by few insects. For this reason, and for their general beauty, ginkgos are excellent urban and shade trees, and are widely planted along many streets.

Ginkgos are also popular subjects for growing as penjing and bonsai; they can be kept artificially small and tended over centuries. Furthermore, the trees are easy to propagate from seed.

Extreme examples of the Ginkgo's tenacity may be seen in Hiroshima, Japanmarker, where six trees growing between 1–2 km from the 1945 atom bomb explosion were among the few living things in the area to survive the blast ( photos and details). While almost all other plants (and animals) in the area were destroyed, the ginkgos, though charred, survived and were soon healthy again. The trees are alive to this day.

The ginkgo leaf is the symbol of the Urasenke school of Japanese tea ceremony. The tree is the national tree of Chinamarker.

Culinary use



The nut-like gametophytes inside the seeds are particularly esteemed in Asia, and are a traditional Chinese food. Ginkgo nuts are used in congee, and are often served at special occasions such as weddings and the Chinese New Year (as part of the vegetarian dish called Buddha's delight). In Chinese culture, they are believed to have health benefits; some also consider them to have aphrodisiac qualities. Japanese cooks add Ginkgo seeds (called ginnan) to dishes such as chawanmushi, and cooked seeds are often eaten along with other dishes.

When eaten by children, in large quantities (over 5 seeds a day), or over a long period of time, the raw gametophyte (meat) of the seed can cause poisoning by MPN (4-methoxypyridoxine). Studies have demonstrated that convulsions caused by MPN can be prevented or terminated with pyridoxine.

Some people are sensitive to the chemicals in the sarcotesta, the outer fleshy coating. These people should handle the seeds with care when preparing the seeds for consumption, wearing disposable gloves. The symptoms are dermatitis or blisters similar to that caused by contact with poison-ivy. However, seeds with the fleshy coating removed are perfectly safe to handle.


Medicinal uses

Extracts of Ginkgo leaves contain flavonoid glycosides and terpenoids (ginkgolides, bilobalides) and have been used pharmaceutically. Ginkgo supplements are usually taken in the range of 40–200 mg per day. Recently, careful clinical trials have shown Ginkgo to be ineffective in treating dementia or preventing the onset of Alzheimer's Disease in normal people.

In memory enhancement

Ginkgo has many alleged nootropic properties, and is mainly used as memoryand concentration enhancer, and anti-vertigo agent. However, studies differ about its efficacy. The largest and longest independent clinical trial to assess ginkgo biloba’s ability to prevent memory loss has found that the supplement does not prevent or delay dementia or Alzheimer’s disease. Some controversy has arisen over the conclusions drawn by some studies that were allegedly funded by a firm which marketed Ginkgo.

In 2002, a long-anticipated paper appeared in JAMA ( Journal of the American Medical Association) titled "Ginkgo for memory enhancement: a randomized controlled trial." This Williams College study, sponsored by the National Institute on Aging rather than Schwabe, examined the effects of ginkgo consumption on healthy volunteers older than 60. The conclusion, now cited in the National Institutes of Healthmarker's ginkgo fact sheet, said: "When taken following the manufacturer's instructions, ginkgo provides no measurable benefit in memory or related cognitive function to adults with healthy cognitive function."...The impact of this seemingly damning assessment, however, was ameliorated by the almost simultaneous publication of a Schwabe-sponsored study in the less prestigious Human Psychopharmacology. This rival study, conducted at Jerry Falwell's Liberty University, was rejected by JAMA, and came to a very different—if not exactly sweeping—conclusion: There was ample evidence to support "the potential efficacy of Ginkgo biloba EGb 761 in enhancing certain neuropsychological/memory processes of cognitively intact older adults, 60 years of age and over."


According to some studies, in a few cases, Ginkgo can significantly improve attention in healthy individuals. Allegedly, the effect is almost immediate and reaches its peak in 2.5 hours after the intake.

In dementia

A 2004 conference paper summarizes how various trials indicate that Ginkgo shows promise in the treatment of Alzheimer's disease, though a 2008 study found it ineffective at treating dementia.

In ongoing studies, a research team led by Luan Luo, PhD, associate professor at the University of Maryland School of Pharmacy, found that giving mice with the human Alzheimer’s gene the ginkgo extract called Egb 761 improved the process of making new nerve cells in part of the brain much affected by the disease. The team found evidence that the protective effect of the extract also could be due to decreasing senile plaques or the clumping of beta-amyloid in the brain tissues.

In other symptoms

Out of the many conflicting research results, Ginkgo extract may have three effects on the human body: improvement in blood flow (including microcirculation in small capillaries) to most tissues and organs; protection against oxidative cell damage from free radicals; and blockage of many of the effects of platelet-activating factor (platelet aggregation, blood clotting) that have been related to the development of a number of cardiovascular, renal, respiratory and central nervous system disorders. Ginkgo can be used for intermittent claudication.

Some studies suggest a link between ginkgo and the easing of the symptoms of tinnitus.

Preliminary studies suggest that Ginkgo may be of benefit in multiple sclerosis, showing modest improvements in cognition and fatigue without increasing rates of serious adverse events in this population.

A study conducted in 2003 by the Department of Dermatology, Postgraduate Institute of Medical Education and Research in Chandigarh, India concluded that Ginkgo is an effective treatment for arresting the development of vitiligo.

Side effects

Ginkgo may have undesirable effects, especially for individuals with blood circulation disorders and those taking anticoagulants such as ibuprofen, aspirin, or warfarin, although recent studies have found that ginkgo has little or no effect on the anticoagulant properties or pharmacodynamics of warfarin. Ginkgo should also not be used by people who are taking certain types of antidepressants (monoamine oxidase inhibitors and selective serotonin reuptake inhibitors) or by pregnant women, without first consulting a doctor.

Ginkgo side effects and cautions include: possible increased risk of bleeding, gastrointestinal discomfort, nausea, vomiting, diarrhea, headaches, dizziness, heart palpitations, and restlessness. If any side effects are experienced, consumption should be stopped immediately.

Allergic Precautions and Contraindications to Use

Ginkgo biloba leaves contain long chain alkylphenols together with the extremely potent allergens, the urushiols (similar to poison ivy). Individuals with a history of strong allergic reactions to poison ivy, mangos, and other usushiol producing plants are more likely to experience an adverse reaction when consuming Ginkgo-containing pills, combinations, or extracts.



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