Ginkgo biloba

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The Ginkgo (Ginkgo biloba), sometimes also 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 examples of a living fossil known. 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 a 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 as of yet uncertain.

Characteristics

Ginkgo fruit 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 brilliant yellow, then fall off within a short space of time. 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 thought to be more than 2,500 years old.

Stem

Ginkgo branches grow in length by apical growth of "long shoots", i.e. ordinary shoots with regularly spaced leaves, as on most trees. These have elongated internodes, and their leaves are often bilobed. From the axils of these leaves, "spur shoots" (also known as short shoots) develop on second-year growth. They have very short internodes (so that several years' growth may extend a spur shoot by only a centimetre or two) and their leaves are ordinarily unlobed. Reproductive structures are only formed from the spur shoots (see picture to above left). In Ginkgos, as in other plants that possess them, spur shoots allow the formation of new leaves in the older parts of the crown. After a number of years, a spur shoot may change into a long shoot, and vice versa.

Leaf

The leaves are unique among seed plants, being fan-shaped with veins radiating out into the leaf blade, sometimes bifurcating but never anastomosing to form a network. Two veins enter the leaf blade at the base and fork repeatedy 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. The leaves 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

Ginkgo ripe fruit and autumn leaf colour

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

Name

The name Ginkgo means "silver apricot" (銀杏 yín xìng) in Chinese. The same name was used in Japan (where Ginkgo had been introduced from China) in the 17th century, but the Japanese pronunciation was ginkyō. This was the name encountered by Engelbert Kaempfer, the first Westerner to see the species, in 1690. The modern Japanese reading is ichō or ginnan (although the kanji text is the same). The modern Chinese name for its shelled seeds is 白果 (bái guǒ), meaning "white fruit".

Prehistory

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, and at the end of the Pliocene Ginkgo fossils disappeared from fossil record everywhere apart from a small area of central China where the modern species survived.

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

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 "discovered" by 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 naturalisation has occurred, with Ginkgos seeding into natural forests.

The seed is edible after removing the ovary pulp, shelling, and after being cooked. Usually only a few are added for a portion enough for ten people. An overdose of the fruit could cause poisoning because the fruit produces hydrogen cyanide as a side product. It is reported that a dozen raw ginkgo fruits are toxic enough to kill a small child, though this has yet to be proven. Some people are sensitive to the chemicals in the sarcotesta. These people should handle the seeds with care when removing the sarcotesta, wearing disposable gloves. The symptoms are a rash or blisters similar to that from poison-ivy.

In some areas, most intentionally planted trees are male stock grafted onto roots propagated from seed, because the male trees will not produce the malodorous fruits (although the seeds within are quite tasty, and a delicacy in Asia). The popular cultivar 'Autumn Gold' is a clone of a male plant.

However, the nuts 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 chawammushi, and cooked seeds are often eaten along with other dishes.

Ginkgo as penjing in the Montreal Botanical Gardens

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 more 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 trees are easy to propagate from seed.

Ginkgos are also popular subjects for growing as penjing and bonsai; they can be kept artificially small and tended over centuries.

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).

Medical uses

Extracts from the Ginkgo leaves contain flavonoid glycosides and ginkgolides and have been used as a pharmaceutical. The extract has many alleged properties, but it is mainly used as memory enhancer and anti-vertigo agents. However, studies differ about its efficacy.

It is commonly added to energy drinks, but it is expensive, so the amount is typically so low it does not produce a noticeable effect, except the placebo effect from 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. It should also not be used by people who are taking the anti-depressant drugs known as monoamine oxidase inhibitors (MAOI) or by pregnant women.

Below are some Ginkgo side effects and cautions:

• gastrointestinal discomfort
• headaches
• may increase risk of bleeding, and should not be used if you have a clotting disorder
• Ginkgo should not be used by pregnant women
• restlessness
• diarrhea
• nausea
• vomiting

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

External links

• The Ginkgo Pages: all aspects, in English, German, French, Spanish and Dutch
• Info by the University of California Museum of Paleontology
• Gymnosperm Database
• Phytochemicals in ginkgo
• The Ginkgo Museum, Weimar, Germany