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GINKGOALES
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GINKGOALES .—This class-designation has been recently proposed to give emphasis to the isolated position of the genus Ginkgo (Salisburia) among the See also:Gymnosperms. Ginkgo biloba, the See also:maiden-See also:hair See also:tree, has usually been placed by botanists in the Taxeae in the neighbourhood of the See also:yew (Taxus), but the proposal by Eichler in 1852 to See also:institute a See also:special See also:family, the Salisburieae, indicated a recognition of the existence of special characteristics which distinguish the genus from other members of the Coniferae. The See also:discovery by the See also:Japanese botanist Hirase of the development of ciliated spermatozoids in the See also:pollen-See also:tube of Ginkgo, in See also:place of the non-motile male cells of typical conifers, served as a cogent See also:argument in favour of separating the genus from the Coniferales and placing it in a class of its own. In 1712 See also:Kaempfer published a See also:drawing of a Japanese tree, which he described under the name Ginkgo; this See also:term was adopted in 1771 by See also:Linnaeus, who spoke of Kaempfer's plant as Ginkgo biloba. In 1797
See also: The venation is like that of many ferns, e.g. Adiantum; the lowest vein in each See also:half of the lamina follows a course parallel to the edge, and gives off numerous branches, which See also:fork repeatedly as they spread in a palmate manner towards the See also:leaf margin. The foliage-leaves occur either scattered on long shoots of unlimited growth, or at the See also:apex of See also:short shoots (spurs), which may eventually elongate into long shoots. The See also:flowers are dioecious. The male flowers (fig. 12), See also:borne in the axil of See also:scale-leaves, consist of a stalked central See also:axis bearing loosely Flowers. disposed stamens; each stamen consists of a slender filament terminating in a small apical scale, which bears usually two, but not infrequently three or four pollen-sacs (fig. 12, C). The axis of the See also:flower is a shoot bearing leaves in the See also:form of stamens. A mature pollen-See also:grain contains a prothallus of 3 to 5 cells (Fig. 13, Pg) ; the exine extends over two-thirds of the circumference, leaving a thin portion of the See also:wall, which on collapsing See also:pro- duces a See also:longitudinal groove similar to the median depression on the pollen-grain of a cycad. The See also:ordinary type of See also:female flower has the form of a long, naked peduncle bearing a single ovule on either See also:side of the apex (fig. 12), the See also:base of each being enclosed by a small, See also:collar-like rim, the nature of which has been vari- ously interpreted. A See also:young ovule consists of a conical nucellus sur- Py rounded by a single in-FIG. 13.—Ginkgo. Apex of Ovule, and tegument terminating as a Pollen-grain. (After Hirase.) two-le ped micrchamber pollen p, Pollen-tube (proximal end). occupies the apex of the c, Pollen-chamber. nucellus; immediately e, Upward prolongation of megaspore. below this, two or more a, Archegonia. archegonia (fig. 13, a) are Pg, Pollen-grain. See also:developed in the upper Ex, Exine. region of the megaspore, each consisting of a large See also:egg-See also:cell surmounted by two See also:neck-cells and a See also:canal-cell which is cut off shortly before fertilization. After the entrance of the pollen-grain the pollen-chamber becomes roofed over by a See also:blunt protuberance of nucellar See also:tissue. The megaspore (embryo-See also:sac) continues to grow after See also:pollination until the greater See also:part of the nucellus is gradually destroyed; it also gives rise to a See also:vertical outgrowth, which projects from the apex of the megaspore as a short, thick See also:column (fig. 13, e) supporting the remains of the nucellar tissue which forms the roof of the pollen-chamber (fig. 13, c). Surrounding the pitted wall of the ovum there is a definite layer of large cells, no doubt representing a tapetum, which, as in cycads and conifers, plays an important part in nourishing the growing egg-cell. The endosperm detached from a large Ginkgo ovule after fertilization bears a See also:close resemblance to that of a cycad; the apex is occupied by a depression, on the See also:floor of which two small holes See also:mark the position of the archegonia, and the outgrowth from the megaspore apex projects from the centre as a short peg. After pollination the pollen-tube grows into the nucellar tissue, as in cycads, and the pollen-grain itself (fig. 13, Pg) hangs down into the pollen-chamber; two large spirally ciliated spermatozoids are produced, their manner of development agreeing very closely with that of the corresponding cells in Cycas and Zamia. After fertilization the ovum-See also:nucleus divides and cell-formation proceeds rapidly, especially in the See also:lower part of the ovum, in which the cotyledon and axis of the embryo are differentiated; the long, tangled suspensor of the cycadean embryo is not found in Ginkgo. It is often stated that fertilization occurs after the ovules have fallen, but it has been demonstrated by Hirase that this occurs while the ovules are still attached to the tree. The ripe See also:seed, which grows as large as a rather small See also:plum, is enclosed by a thick, fleshy envelope covering a hard woody See also:shell with two or rarely three longitudinal keels. A papery remnant of nucellus lines the inner See also:face of the woody shell, and, as in cycadean seeds, the apical portion is readily separated as a cap covering the See also:summit of the endosperm. The See also:morphology of the female flowers has been variously interpreted by botanists; the peduncle bearing the ovules has been described as homologous with the petiole of a foliage-leaf and as a shoot-structure, the collar-like envelope at the base of the ovules being referred to as a second integument or arillus, or as the representative of a carpel. The See also:evidence afforded by normal and abnormal flowers appears to be in favour of the following See also:interpretation: The peduncle is a shoot bearing two or more carpels. Each ovule is enclosed at the base by an envelope or collar homologous with the lamina of a leaf ; the fleshy and hard coats of the nucellus constitute a single integument. The stalk of an ovule, considerably reduced in normal flowers and much larger in some abnormal flowers, is homologous with a leaf-stalk, with which it agrees in the structure and number of vascular bundles. The facts on which this description is based are derived partly from anatomical evidence, and in part from an account given by a Japanese botanist, Fujii, of several abnormal female flowers; in some cases the collar at the base of an ovule, often described as an arillus, is found to pass gradually into the lamina of a leaf bearing marginal ovules (fig. I B). The occurrence of more than two ovules on one peduncle is by no means rare; aparticularly striking example is described by Fujii, in which an unusually thick peduncle bearing several stalked ovules terminates in a scaly bud (fig. 14, A, b). The frequent occurrence of more than two pollen-sacs and the equally common occurrence of additional ovules have been regarded by some authors as evidence in favour of the view that ancestral types normally possessed a greater number of these See also:organs than are usually found in the See also:recent species. This view receives support from fossil evidence. Close to the apex of a shoot the vascular bundles of a leaf make their See also:Anatomy. See also:appearance as See also:double strands, and the leaf-traces in the upper part of a shoot have the form of distinct bundles, which in the older part of the shoot form a continuous See also:ring. Each double leaf-trace passes through four internodes before becoming a part of the See also:stele; the double nature of the trace is a characteristic feature. Secretory sacs occur abundantly in the leaf-lamina, where they appear as short lines between the See also:veins; they are abundant also in the cortex and See also:pith of the shoot, in the fleshy integument of the ovule, and elsewhere. The secondary See also:wood. of the shoot and See also:root conforms in the See also:main to the coniferous type; in the short shoots the greater breadth FIG. 14.—Ginkgo. Abnormal female of the medullary rays in Flowers. A, Peduncle; b, scaly bud; the more See also:internal part of B, leaf bearing marginal ovule. (After the xylem recalls the Fujii.) cycadean type. The secondary phloem contains numerous thick-walled See also:fibres, parenchymatous cells, and large See also:sieve-tubes with plates on the radial walls; swollen parenchymatous cells containing crystals are commonly met with in the cortex, pith and medullary-See also:ray tissues. The wood consists of tracheids, with circular bordered pits on their radial walls, and in the See also:late summer wood pits are unusually abundant on the tangential walls. A point of anatomical interest is the occurrence in the vascular bundles of the cotyledons, scale-leaves, and elsewhere of a few centripetally developed tracheids, which give to the xylem-strands a mesarch structure such as characterizes the foliar bundles of cycads. The root is diarch in structure, but additional protoxylem-strands may be See also:present at the base of the main root; the pericycle consists of several layers of cells. This is not the place to discuss in detail the past See also:history of Ginkgo (see See also:PAL AEOBOTANY : Mesozoic). Among Palaeozoic genera there are some which See also:bear a close resemblance to the recent type in See also:Geological the form of the leaves; and petrified Palaeozoic seeds, history. almost identical with those of the See also:maidenhair tree, have been described from See also:French and See also:English localities. During the Triassic and See also:Jurassic periods the genus Baiera--no doubt a representative of the Ginkgoales—was widely spread throughout Europe and in other regions; Ginkgo itself occurs abundantly in Mesozoic and See also:Tertiary rocks, and was a common plant in the See also:Arctic regions as elsewhere during the Jurassic and Lower Cretaceous periods. Some unusually perfect Ginkgo leaves have been found in the See also:Eocene leaf-beds between the See also:lava-flows exposed in the cliffs of See also:Mull (fig. II). From an evolutionary point of view, it is of interest to See also:note the occurrence of filicinean and cycadean characters in the maidenhair tree. The leaves at once invite a comparison with ferns; the numerous long hairs which form a delicate woolly covering on young leaves recall the hairs of certain ferns, but agree more closely with the long filamentous hairs of recent cycads. The spermatozoids constitute the most striking See also:link with both cycads and ferns. The structure of the seed, the presence of two neck-cells in the archegonia, the late development of the embryo, the partially-fused cotyledons and certain anatomical characters, are features common to Ginkgo and the cycads. 'The maidenhair tree is one of the most interesting survivals from the past; it represents a type which, in the Palaeozoic era, may have been merged into the extinct class Cordaitales. Through the succeeding ages the Ginkgoales were represented by numerous forms, which gradually became more restricted in their distribution and fewer in number during the Cretaceous and Tertiary periods, terminating at the present See also:day in one solitary survivor. Additional information and CommentsThere are no comments yet for this article.
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