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Ginkgo

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Ginkgo

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Ginkgo
Conservation status: Endangered[1]
Ginkgo leaf
 
Ginkgo leaf
Scientific classification
Kingdom: Plantae
 
Division: Ginkgophyta
 
Class: Ginkgoopsida
 
Order: Ginkgoales
 
Family: Ginkgoaceae
 
Genus: Ginkgo
 
Species: G. biloba
 
Binomial name
Ginkgo biloba

The Ginkgo (Ginkgo biloba), frequently misspelled as "Gingko", and sometimes known as the Maidenhair Tree, is a unique tree with no close living relatives. It is classified in its own division, the Ginkgophyta, comprising the single class Ginkgoopsida, order Ginkgoales, family Ginkgoaceae, genus Ginkgo and just the one species. It is one of the best known examples of a living fossil. In the past it has also been placed in the divisions Spermatophyta or Pinophyta. Ginkgo is a gymnosperm (as opposed to an angiosperm), meaning "naked seed"; its seeds are not protected by an ovary wall and hence, the berry-like structures produced by female ginkgo trees are technically not fruit.

For centuries it was thought to be extinct in the wild, but is now known to grow wild in at least two small areas in Zhejiang province in eastern China, in the Tian Mu Shan Reserve. However, as this area has known human activity for over a thousand years, the wild status of ginkgos there is uncertain.

Characteristics

Ginkgo seeds and leaves
Ginkgo seeds and leaves

Habit

Ginkgos are medium-large deciduous trees, reaching 20–35 m tall (some specimens in China being over 50 m), with an often angular crown and long, somewhat erratic branches. They are 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 amazing disease resistance, insect-resistant wood and the ability to form aerial roots and sprouts means that ginkgos are very long-lived, with some specimens claimed to be more than 2,500 years old; a 3,000 year-old ginkgo is reported in Shandong province in China (Lewington and Parker, 183).

Some old Ginkgos produce aerial roots, known as chichi (Japanese; "nipples") or zhong-ru (Chinese), which form on the undersides of large branches and grow downwards. Chichi growth is very slow, and may take hundreds of years to occur. The function, if any, of these thick aerial roots is unknown.

Stem

Ginkgo mature seeds and autumn leaf colour
Ginkgo mature seeds and autumn leaf colour

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 internodes (so that several years' growth may only extend them by a centimeter or two) and their leaves are ordinarily 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 picture to above left—seeds and leaves can be viewed 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

A closeup of a Ginkgo leaf
A closeup of a Ginkgo leaf

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 5-10 cm (rarely to 15 cm) 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. During summer the leaves are a deep green, turning to brilliant yellow in the fall. They generally remain yellow for a time, then suddenly drop most of their leaves in what can seem like overnight.

Reproduction

autumn leaves and seeds
autumn leaves and seeds

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.

Ginkgo pollen cones
Ginkgo pollen cones

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 outer layer (the sarcotesta) is light yellow-brown, soft, and fruit-like. It is plum-like and attractive, but the seed coat contains butanoic acid and smells like rancid butter (which contains the same chemical) when fallen on the ground. Beneath the sarcotesta is the hard sclerotesta and a papery endotesta and nucellus.

The fertilization of ginkgo seeds is by motile sperm; similar to cycads, ferns, mosses and the algae. It is a large sperm of about 250-300 micrometres, similar to the cycad which is a little larger. It was first discovered by the Japanese botanist Sakugoro Hirase in the early 1900s. The sperm has 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. But it has 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, [2] [3] [4] embryos ordinarily occur in seeds just before and after they drop from the tree. [5]

Female gametophyte, dissected from a seed freshly shed from the tree, containing a well-developed embryo
Female gametophyte, dissected from a seed freshly shed from the tree, containing a well-developed embryo

Name

The name ginkgo means "silver apricot" (銀杏, pinyin: yínxìng) in Chinese. The same characters are used in Japanese and Korean (where the ginkgo had been introduced from China). The Japanese pronunciation is ichō while the Korean equivalent is eunhang, both of which appear to be a loan from Chinese, though this is not certain (from the entry in the dictionary Kōjien). The Japanese characters used to write ginkgo look as though they could be read ginkyō, and this was the name Engelbert Kaempfer, the first Westerner to see the species in 1690, wrote down in his Amoenitates Exoticae (1712). However, his y was misread as a g, and the misspelling stuck.

In modern Japanese, the characters are read either ichō (meaning the tree) or ginnan (meaning the seed); this latter reading appears to be based on the renjō (i.e., liaison) reading of the characters. The modern Chinese name for its shelled seeds is 白果 (Mandarin bái guǒ), meaning "white fruit".

Prehistory

Fossil Ginkgo leaves from the Jurassic of England
Fossil Ginkgo leaves from the Jurassic of England

The Ginkgo is a living fossil, with fossils recognisably related to modern Ginkgo from the Permian, dating back 270 million years. They diversified and spread throughout Laurasia during the middle Jurassic and Cretaceous, but became much rarer thereafter. By the Paleocene, Ginkgo adiantoides was the only Ginkgo species left in the Northern Hemisphere (but see below) with a markedly different (but not well-documented) form persisting in the Southern Hemisphere, and at the end of the Pliocene Ginkgo fossils disappeared from the fossil record everywhere apart from a small area of central China where the modern species survived. It is in fact doubtful whether the Northern Hemisphere fossil species of Ginkgo can be reliably distinguished; given the slow pace of evolution in the genus, there may have been only 2 in total; what is today called G. biloba (including G. adiantoides), and G. gardneri from the Paleocene of Scotland.

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 just as well have simply 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. The occurrence of G. gardneri, it seems a Caledonian mountain endemic, and the somewhat greater diversity on the Southern Hemisphere, suggests that old mountain ranges on the Northern Hemisphere could hold other, presently undiscovered, fossil Ginkgo species. Since the distribution of Ginkgo was already relictual in late prehistoric times, the chances that ancient DNA from subfossils can shed any light on this problem seem remote. 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 (being restricted to light soils around rivers), and a low population density.

Ginkgophyta fossils have been classified in the following families and genera:

  • Ginkgoaceae
    • Arctobaiera
    • Baiera
    • Eretmophyllum
    • Ginkgo
    • Ginkgoites
    • Sphenobaiera
    • Windwardia
  • Trichopityaceae
    • Trichopitys

Ginkgo has been used for classifying plants with leaves that have more than four veins per segment, while Baiera for those with less 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

Ginkgos along Harlem Avenue in Riverside, Illinois
Ginkgos along Harlem Avenue in Riverside, Illinois

Ginkgo has long been cultivated in China; 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.

Ginkgo tree in autumn
Ginkgo tree in autumn

In some areas, notably the United States, 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.

The Ginkgo has the intriguing distinction of being one of the world's most urban-tolerant trees, often growing where other trees cannot survive. Some claim that only one tree species, the Tree-of-heaven, is as urban-tolerant. Ginkgos 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. The ginkgo is the official tree of the city of Kumamoto, and two leaves form the symbol of the University of Tokyo, the main campus of which is famous for its numerous ginkgos.

Ginkgo leaves painted on an asphalt walkway to guide tourists to a ginkgo forest in Dongducheon, South Korea.
Ginkgo leaves painted on an asphalt walkway to guide tourists to a ginkgo forest in Dongducheon, South Korea.

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, Japan, where four 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 & details). While almost all other plants (and animals) in the area were destroyed, the ginkgos, though charred, survived and were healthy. The trees are alive to this day.

Culinary use

The nut-like gametophytes inside the seeds are esteemed in and outside of Asia, and are a traditional Chinese food (e.g. congee, often served at weddings), and are believed to have health benefits; some also consider them to have aphrodisiac qualities. Japanese cooks add Ginkgo seeds to dishes such as chawanmushi, and cooked seeds are often eaten along with other dishes. The seeds are available canned, sold as "White Nuts", and can be found in many Asian food stores in the West. Usually only a few are added for a portion enough for ten people.

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). MPN is heat-stable. 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.

Ginkgo as penjing in the Montreal Botanical Garden
Ginkgo as penjing in the Montreal Botanical Garden

Medical uses

The extract of the Ginkgo leaves contains flavonoid glycosides and ginkgolides and has been used pharmaceutically. It has many alleged nootropic properties, and is mainly used as memory enhancer and anti-vertigo agent. However, studies differ about its efficacy.

Out of the many conflicting research results, there seem to be basically three effects of Ginkgo extract on the human body: it improves blood flow (including microcirculation in small capillaries) to most tissues and organs; it protects against oxidative cell damage from free radicals (antioxidant); and it blocks many of the effects of PAF (platelet aggregation, blood clotting) that have been related to the development of a number of cardiovascular, renal, respiratory and CNS (Central Nervous System) disorders. Ginkgo can be used for intermittent claudication.

A 2004 conference paper [6] summarises how various trials indicate that Ginkgo shows promise in the treatment of Alzheimer's disease, although further study is needed.

Ginkgo is commonly added to energy drinks, but the amount is typically so low it does not produce a noticeable effect, except perhaps via a placebo effect from Ginkgo being listed on the label.

Side effects

Ginkgo may have some undesirable effects, especially for individuals with blood circulation disorders and those taking anti-coagulants such as aspirin and warfarin, although recent studies have found that ginkgo has little or no effect on the anticoagulant properties or pharmacodynamics of warfarin[7][8]. Ginkgo should also not be used by people who are taking monoamine oxidase inhibitors (MAOI) 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, and restlessness.

If any side effects are experienced consumption should be halted immediately. Ginkgo supplements are usually taken in the range of 40–200 mg per day. If the side effects continue usage should be stopped completely.

See also

References

  1. ^ Sun (1998). Ginkgo biloba. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 11 May 2006. Listed as Endangered (EN B1+2c v2.3)
  2. ^ Brief Notes on Ginkgo biloba
  3. ^ Ginkgoales: More on Morphology
  4. ^ Laboratory IX -- Ginkgo, Cordaites, and the Conifers
  5. ^ Ben F. Holt, Gar W. Rothwell. Is Ginkgo biloba (Ginkgoaceae) Really an Oviparous Plant? American Journal of Botany, Vol. 84, No. 6 (Jun., 1997) , pp. 870-872
  6. ^ L. Witkam and I. Ramzan (2004). "Ginkgo biloba in the treatment of Alzheimer’s disease: A miracle cure?". From Cell to Society. full text pdf  Conference page.
  7. ^ Xuemin Jiang et al (2005). Effect of ginkgo and ginger on the pharmacokinetics and pharmacodynamics of warfarin in healthy subjects. British Journal of Clinical Pharmacology 59 (4): 425–432.
  8. ^ Ernst E, Canter PH, Coon JT (2005). Does ginkgo biloba increase the risk of bleeding? A systematic review of case reports. Perfusion 18: 52–56.
4. Lewington, A., & Parker, E. (1999). Ancient Trees. London: Collins & Brown Ltd. ISBN 1-85585-704-9.

External links


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This guide is licensed under the GNU Free Documentation License. It uses material from the Wikipedia.


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