Sunday, 3 February 2019

Ginkgoales Part 1


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 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. 


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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