Ginkgoales
Ginkgoales is an ancient group which appeared in the Permian, and was well-represented and nearly worldwide in distribution in the Mesozoic. Their maximal diversity occurred during the Middle Jurassic, and remains have been collected from many countries: Alaska, Greenland, Scandinavia, Siberia, Mongolia, England, Europe, China, Japan, Australia, New Zealand, India, tip of South Africa and South and North America. These plants flourished for over 150 million years. By the beginning of the Oligocene (Tertiary), only 2 out of ca. 19 genera survived; Ginkgo adiantoides is one of these. This species became extinct by the Pliestocene (Tertiary), and Ginkgoales has been represented by the extant Ginkgo biloba-the "living fossil". It appeared in the Jurassic, and became extensively distributed in the Tertiary. In later periods, it was abundant in the more northern latitudes. At present, its natural occurrence is restricted to a small inaccessible region in southeastern China.
Fossil Taxa
Abundant compression-impression leaf remains have been collected,the reproductive organs are meagre. There are about seven or eight Mesozoic leaf genera included in Ginkgoales (Harris 1935, 1974). Ginkgoites and Baiera have the maximal number of species. These two taxa are indistinguishable in their cuticular and stomatal characteristics. However, Baiera lacks a petiole and its lamina is comparatively more wedge-shaped, than that of Ginkgoites. The latter has a distinct petiole with two traces which diverge into the basal edge of a bilobed lamina. The fossil leaves, which cannot be distinguished from those of the extant Ginkgo, are included in this taxon, but the species are different. Those leaves which can be distinguished by their morphological and anatomical characteristices (size and shape of epidermal cells, distribution of stomata, structure of subsidiary cells, mesophyll and distribution of resin bodies) are placed in the genus Ginkgoites.
There are very few verified reports of reproductive structures. Numerous dispersed ovules of Allicospermum (Harris 1935) -similar to those of cycads and G. biloba-are associated with Jurassic and Cretaceous Ginkgo-like remains. Kp,rkenia is an ovulate fructification from the Cretaceous of Argentina (Archangelsky 1965). It is a short stalk crowded with 100 or more ovules associated with Ginkgoites tigrensis. Paired ovules joined by a pad of tissue with Ginkgo-like stomata have been reported from Yorkshire Jurassic beds (Harris 1976), where abundant Ginkgoites huttoni occurs. In the same beds, small pollen-bearing catkins are present. Each consists of a stalk (ca. 5 mm long) to which are attached rather lax stalks with pairs of microsporangia at the tips. The pollen grains are monocolpate like those of G. biloba. The male organs of Trichopitys are not known. However, Sphenobaiera furcata (Triassic) bears clusters of microsporangia at the branch tips of a bifurcating axis, which are borne in tum on short shoots, along with leaves similar to Trichopitys.
Ginkgoaceae
Ginkgoaceae is a monotypic family. The deciduous leaves are fan-shaped with parallel veins. The tree is dioecious; male flowers are catkin-like while the female is long-stalked with (usually) two ovules. The male gametes are motile, and the fruit is drupaceous.
Morphology
Ginkgo biloba is a tree more than 30 m high and exceeds 1.5 m in diameter. It resembles a conifer in its general habit. The crown becomes broad, irregular with age, and shows a variable pattern of branching. The main axis branches profusely. There are two kinds of shoots: (a) long shoots which elongate rapidly and bear scattered leaves, and (b) dwarf shoots which grow slowly and bear a terminal cluster of leaves; the older portion is covered with leaf scars of previous years .Bud scales cover young spur (dwarf) shoots. Frequently, the dwarf shoots become more active and tum into a long shoot, while (more rarely) the terminal growth of a long shoot may be retarded and resemble a lateral spur shoot (Gunckel and Wetmore 1946). This suggests that there is not much fundamental difference between them. The apical meristems of the two types of shoots are also essentially similar; the difference between them depends on the duration of cell division and cell elongation in the stem tissues derived from the shoot apex (Foster 1938). There is experimental evidence to show that the dwarf shoots are formed due to the inhibitory effects of auxins produced by the tissues of the long shoots (Gunckel et al. 1949).
The shape and venation of the deciduous foliage leaves are unique. The petiole is long, smooth, black and slender, traversed by two collateral vascular bundles. The lamina is broadly wedge-or fan-shaped, variously lobed, and the venation is conspicuously dichotomous.
The leaves resemble those of Adiantum (maidenhair fern) in form and venation, hence the popular name maidenhair tree. The old leaves are shed in autumn, when they change colour to a golden yellow; the new leaves appear in spring. There is considerable difference in the lobing of the leaves on the same tree. They may be nearly entire or two-lobed due to a conspicuous, often deep, apical notch. They are mostly bilobed on long shoot and entire on short shoot. In seedlings the leaves have several notches which give a palmatifid appearance (as in the extinct taxa).
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