Ginkgos

Ginkgos tree
Ginkgos tree
The ginkgos, phylum Ginkgophyta, constitute one of four phyla of the gymnosperms in the kingdom Plantae. Ginkgo biloba, the maiden hair tree, is the only living representative of the ginkgo family, Ginkgoaceae, a group of plants that have lived for millions of years and are identified by an abundant fossil record.

The ginkgo is a hairless, deciduous tree with a straight trunk and pyramid-shaped foliage usually sparsely branched when young, becoming denser with age. Leaves are fan-shaped, 2 to 3 inches (5 to 7.5 centimeters) across, sometimes divided into two lobes.

The ginkgo normally reaches heights of 80 to 100 feet (24 to 30meters) and under favorable conditions grows to 125 feet (38 meters) or more. The bark is reddish-gray and corky, with irregular, wide fissures dividing rough plates. On old trees, the bark becomes gray, rough, and deeply furrowed.

Considerable diversity in branching habit occurs, sometimes with one side of the tree having erect branching and the other side spreading limbs. Young trees send out straight branches at a skyward angle and, until maturity, the sparse branching gives the tree an erratic appearance. Upon maturity, the branches round out and become widespread, yet retain an uneven crown.


As in many conifers, the long branches (shoots) and short, spurlike shoots of Ginkgo biloba are easily distinguished. The leaves are spirally arranged on both types but widely spaced on the long shoots, with leaves in crowded, rosettelike clusters on the short shoots. Branchlets (twigs) have a horizontal or drooping habit and are occupied with short, spurlike shoots.

These shorter shoots increase in length only a fraction of an inch (2.5 centimeters) per year and may produce clusters of leaves annually for many years before abruptly lengthening out into long shoots bearing scattered leaves. The fan-shaped leaf has a marked resemblance to the fronds of the maidenhair fern, thus the common name: maidenhair tree. However, in its native China it is commonly called ducks-foot tree, also based on leaf shape.

The leaves, which grow on slender stalks up to 3 inches (7.5 centimeters) long, have numerous veins radiating out from the base to an irregularly notched leaf margin.

Ginkgos plant
Ginkgos plant

There is no central midrib vein on the somewhat leathery, textured leaf. Stomata (breathing pores in the leaves) occur on both the upper and lower surfaces of the leaves. The leaves emerge yellow-green in spring but turn green toward midsummer and become golden in autumn.

Leaves on vigorous young trees can grow up to 6 to 8 inches (15 to 20 centimeters) in width. There is a morphological distinction between leaves of long branches and short shoots, with the leaves of long branches generally bilobate to fourlobed and those of short shoots only fan-shaped to bilobate.

Elliptical, naked seeds resembling a small plum appear on female trees in early spring. Seeds range from 0.75 to 1 inch long (1.9 to 2.5 centimeters) and are covered by a thin, yellowish-orange, fleshy outer wall enveloping a woody shell which contains an edible kernel in the shell interior.

When falling to the ground in autumn, the seed covering begins to diminish in thickness over several months, giving off the vile odor of butanoic and hexanoic acids (butter and Romano cheese fatty acids), and is eventually lost from most seeds. Ginkgo biloba wood is light, brittle, yellowish in color, and of little value. It is used as a base wood in highly lacquered furniture and small carved items.

Reproduction

The ginkgo is dioecious:Male and female reproductive structures are borne on separate trees. The male reproductive structures appear in May and are inch-long, catkinlike structures bearing numerous paired, pollen-bearing organs.

The pollen grains are similar to the elliptical grain of cycads. The pollen organs and ovules are confined to the short shoots of each ginkgo tree and arise in the leaf axils or inner bud scales.

Ginkgos fruits
Ginkgos fruits
The female reproductive structure consists of a long stalk, bearing on each side an erect, naked ovule, which is surrounded at the base with a collarlike rim. The paired ovules are borne in groups of two to ten. The three-layered ovules include a fleshy exterior layer (sarcotesta), an inner flesh, and a stony shell (sclerotesta) between the two.

This three-layered structure is called the integument. The nucellus (the central cellular mass of the body of the ovule containing the embryro) is mostly free from the surrounding integument, except at its base, where it develops a pollen chamber at its apex.

Similar to the cycads, the gingko reproduces by means of flagellated sperm cells, which are carried by the wind-borne pollen to the female reproductive structures within the ovule. In the ginkgo, the vascular system is weakly developed and consists of a pair of braided bundles in the interior fleshy layer of the integument.

Upon maturation of the microgametophyte (male gametophyte), pollen tubes are produced, as are large,motile sperm cells similar to those of the cycads. Megagametophyte (female gametophyte) development is similar to that in cycads as well.

Natural Regeneration

Studies into the seedling development of Ginkgo biloba reveal a unique mechanism of clonal regeneration that may help explain the species’ long survival in the natural setting. The organ of clonal regeneration in the gingko is called the basal chichi. These organs are part of aggregates of suppressed shoot buds and are located in embryonic tissue of Ginkgo biloba seedlings.

When damage occurs to the seedling axis, one of these subsurface buds grows down from the tree trunk to form a woody, stemlike basal chichi. Regeneration of Ginkgo biloba by basal chichi promotes survival of the tree in the forests of China today and may have been a factor in the protracted survival of the order since the Mesozoic era.

Habitat and Range

Ginkgos leaves
Ginkgos leaves
Ginkgo biloba, a distinctive tree suited for use in singular or in group plantings on lawns or along streets, is widely cultivated in all temperate zones. It prospers in moderately moist, fertile soil in humid, temperate regions. It is extremely resistant to disease and pests, and it is highly tolerate of smoke, dust, wind, and ice.

Ginkgo biloba is apparently native in eastern China, with documented semi wild trees growing on the west peak of Tian Mu Mountain in the Tian Mu Reserve, Zhejiang Province. Ginkgo biloba is planted in the eastern United States, Europe, and along the Pacific coast.

Fossil Record

Among the plants, Ginkgo biloba is probably the best-known example of a “living fossil.” Although the ancestors to the order Ginkgoales date to the Paleozoic era, it was at the close of the Triassic period when they became a dominant part of the Mesozoic flora. During the Jurassic period, especially the middle Jurassic, Ginkgoales reached zenith numbers of species and its widest distribution.

Jurassic and Cretaceous fossil localities reveal circumpolar Ginkgophytes sites, including Alaska, Greenland, Zemlya Frantsa Iosifa (Franz Joseph Land), and Mongolia, with the Siberian locations especially productive. Southern Hemisphere Ginkgoales localities include Patagonia at the southern tip of South America, South Africa, India, Australia, and New Zealand.

European fossil sites are known in England, Scotland, Germany, Italy, Hungary, Turkestan, and Afghanistan. Western Canada and the United States have Ginkgoites leaf remains from the Upper Mesozoic and Lower Tertiary deposits.

The presence of Ginkgophytes in high northern latitudes during the Early Cretaceous period and its presence in southern latitudes, such as Argentina, during the Jurassic, suggests that the dispersal of the plant was from the southern to northern latitudes during the Upper Mesozoic era.

During the Tertiary period, the decline of the Ginkgophytes was evident from the presence of only two of nineteen species remaining in the fossil record. One of the remaining two species is the Ginkgo adiantoides, which declined sharply during the Oligocene period.

This decline continued into the Miocene period in North America, with Ginkgo adiantoides disappearing from the fossil record at the end of the Miocene. Ginkgo adiantoides did continue into the Pliocene in Europe, however. Since the Pliocene, the fossil record indicates that Ginkgoales have been represented by the extant living fossil, Ginkgo biloba.

Researchers propose that the decline of Ginkgophyta was a result of competition from the angiosperms (flowering plants) for similar plant habitats. Also, the Ginkgophytes became more restricted to northern temperate forests in the Tertiary period. When glaciation occurred in these areas during the Pleistocene, these forests were destroyed by climate change.

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