Ginkgo

Ginkgo is an unusual nonflowering plant with the only surviving species (Ginkgo biloba) of an entire taxonomic division - Ginkgophyta. Its survival to modern times is largely accredited to human cultivation of the Ginkgo, and our many uses of it including food, medicine and decoration. It was even believed to be extinct by European botanists, until 1690 when traders discovered Ginkgoes being grown and maintained in large numbers at monasteries in China. Since then, Ginkgoes can be seen all over the world, as it is theorized they once could be. From their unique leaf shape that earned Gingko biloba its name, to the cultural ties to Asia, the Ginkgo is a living piece of history. Thought to date back to the early Permian period by palaeobotanists, the Ginkgo biloba is considered a living fossil of a very large and diverse lineage. Hardy trees, they can resist pests, pollution and fungus and live for over one thousand years. Usually only males are cultivated though, as the female trees drop malodorous fruit, that are even illegal in some areas for their ability to cause allergic reactions and even burn the skin.

Anatomy
The Ginkgo biloba appears rather small in its youth, and may only be a few feet tall. Once a tree reaches maturity, however, it may become very wide-spreading, with a thick trunk and dense, ascending branches that appear to spread from the trunk almost diagonally, full of clusters of very distinct leaves. The size of the tree and arrangement of branches often depends on the age of the tree and whether it is male or female. Young male trees are described as either 'Mariken' for having compact, pendulous branches, or 'Saratoga' which features distinctly upright branches and often leaves of a brighter hue. Some female trees may be described as 'Tremonia' as they have a much narrower form in youth than the broad-branching males but grow up to thirty feet when starting their growth to maturity. While tolerant of most climates and soil, some trees may take over ten years to grow four feet, but a Ginkgo can reach up to eighty feet tall when fully mature. The bark of the trunk is thick with slight fissures and has a gray-black color.

The leaves themselves, are a feature almost unique to the Ginkgo. They resemble a two-lobed fan shape with a deep central notch on small spur shoots. They are a crisp green on both sides, with scalloped margins. Each leaf is decorated with fine, parallel veins that radiate outwards from the base of the leaf near the stem. The leaves are said to resemble the maidenhair fern, thus earning the Ginkgo its other name, the maidenhair tree. Leaves are often clustered with up to five on small shoots. Both small and long shoots are produced, which arrange the leaf clusters in a spiral pattern along the branches. The leaves are referred to as tubifolia, for their compact, tube shape when sprouting.

Reproduction
Ginkgoes are dioecious, meaning that there are male and female trees, with each producing a specific gamete. Male trees produce pollen cones and sporophylls, with pollen sacs. The cones appear in small clusters, hanging from the spur shoots and usually begin to appear in the spring. Each cone is anywhere from 3/4 to one inch (2 to 2.5 cm) around, and develop on dwarf shoots. The pollen relies in the wind to transfer pollen grains to a female tree, which still depend on water to reach an ovule.

Females do not produce cones, but instead form two ovules on the end of a stalk. After pollination, the ovules will turn to small seeds. The seed itself is composed primarily of three main layers. A fleshy, outer sarcotesta, a hard inner layer known as the sclerotesta and a thin endotesta. The sclerotesta is commonly thought of as the shell of the seed. Most seeds are dispersed by animals who may eat them, where the seed will pass through the gut of the animal. Due to the difficulty of germinating ginkgo seeds by regular cultivation, it is speculated that passing through the gut may be vital in some way to their germination. As this is extremely difficult to study, it is still just speculation.

The ovules swell up around the time pollen is released by a neighboring male tree. In each ovule is a tiny cavity within a large space of reserve, which is filled with liquid. This small space, known as the pollinic room, presents a tiny opening, or micropyle, to produce a viscous drop of liquid. When a grain of pollen passes by the opening, it is trapped in the liquid and the micropyle shuts. Once in the ovum, motile sperm are released from the pollen grain. Ginkgo seeds mature and fall in the autumn, though it may take a tree up to forty years to reach reproductive maturity. The seeds scatter the ground, and are fleshy and fruit-like in appearance. However appealing they may appear, they are notorious for the putrid odor they emit when cracked or damaged. This is due to the high presence of butyric acid, which may also cause certain allergic reactions in people who try to handle the pulp left behind by the decaying sarcotesta. For this reason, males are the most commonly cultivated.

Ecology
Ginkgoes are deciduous trees as they will drop their leaves in the winter, after turning a brown tint in the fall. While they can be found in cities all over the world, they thrive mostly in areas with moderate rainfall. Mediterranean climates with wet winters and hot summers are close to ideal for this tree. They are also commonly found in the wild in deciduous forests, and in soil composed mostly of loess, but will generally tolerate a wide pH range. Ginkgo biloba are not picky about their environment, and are very durable trees, capable of surviving many pests and diseases caused by pollution or fungus. Due to the tree's highly resistant nature, Ginkgo biloba can live for almost one thousand years.

Ginkgoales is a group of some of the oldest trees on the planet. The Ginkgo biloba is considered a living fossil. Ginkgo fossils have been found all over the world and many of them appear to have been of many different species. The biggest difference between the extinct species and the living fossil Ginkgo biloba, seems to have been a difference in the number of reproductive versus vegetative organs. Ginkgophyte leaves also varied greatly, and numerous fossil discoveries of species believed to have that of ginkgoes show a very large diversity in leaf structure. In fact, the modern Ginkgo biloba gets its name from the Latin word meaning "two-lobed," in reference to the bisection of the leaf. The oldest ovulate organ of a ginkgo was found in China, that of G. yimaensis. Many different botanists have proposed theories for what appears to have been a mass extinction of numerous ginkgo species. Due to the fact that they were most associated with, and introduced to the west by, China and Japan, it is suspected that G. biloba may have remained there, surviving mostly by human cultivation.

Much of the reason they can be seen cultivated in cities all over the world today, may be that they were cultivated and kept for many years in China.In fact, the name for the Ginkgo comes from the Chinese name yin-kuo or silver fruit. Ginkgoes were not known to European botanists, until the discovery by Engelbert Kaempfer, a surgeon of the Dutch East India Company in 1690, with the first text on Ginkgoes being published later in 1712. It is still widely believed that the continued survival of Ginkgo biloba today is mostly, if not entirely, due to its cultivation in China, where its products were used medicinally.

Medicinal Uses of Ginkgo
While the Ginkgo has been used in cooking as well as traditional medicine, not until recently have researchers fully studied the affects of Ginkgo biloba related drugs. Claims have been made from enhancing memory and various other cognitive functions in adults over 60, as well as preventing the onset of Alzheimer's and dementia. In the Journal of the American Medical Association's publication Ginkgo biloba for Prevention of Dementia: A Randomized Controlled Trial," the results and data of carious experiments with Ginkgo products on the prevention of dementia was collected.

Over the course of about eight years, from 2000-2008, five US medical centers had over 3,069 volunteers aged 75 or older. Each of them had normal cognition, or mild cognitive impairment. Half were given a twice daily dose of 120 mg G. Biloba extract, the other half a twice daily dose of placebos. A study entry was made every six months to address any cases of incident dementia. Of the 3,069 initial volunteers (2587 with normal cognition, 482 with MCI) five-hundred twenty-three developed dementia. 277 of them, the majority, were taking the G. biloba supplements but developed dementia anyway. 92% of the cases of dementia were diagnosed as possible or probable cases of Alzheimer's disease. The rates of developing dementia were 3.3 percent for every 100 volunteers taking the G. Biloba supplement, but only 2.9 percent for every 100 volunteers who were only taking placebos. Due to the results, the Journal of the American Medical Association concludes that the dosage of G. biloba was not effective in preventing dementia.