Helping San Diego, California and beyond since 1997.
Do you need
volunteer, community service, work, military or court hours?
|
|
Helping San Diego, California and beyond since 1997.
|
|
Click here and add this page to your favorites!
|
Encyclopedia Britannica - Main :: GEO-GNU |
|
|
GINKGOALES .This class-designation has been recently proposed to give emphasis to the isolated position of the genus Ginkgo (Salisburia) among the Gymnosperms. Ginkgo biloba, the maiden-hair tree, has usually been placed by botanists in the Taxeae in the neighbourhood of the yew (Taxus), but the proposal by Eichler in 1852 to institute a special
special
Smith proposed to use the name Salisburia adiantifolia in preference to the " uncouth " genus Ginkgo and " incorrect " specific term biloba. Both names are still in common use. On account of the resemblance of the leaves to those of some species of Adiantum, the appellation maiden-hair tree has long been given to Ginkgo biloba. Ginkgo is of special interest
of temples in the Far East, and often cultivated in North America and Europe. Ginkgo biloba, which may reach a height of over 30 metres, forms a tree of pyramidal shape with a smooth grey bark. The leaves (figs. to and II) have a long, slender petiole terminating in a fan-shaped lamina, which may be entire, divided by a median incision into two wedge-shaped lobes, or subdivided into several narrow segments. The venation is like that of many ferns, e.g. Adiantum; the lowest vein in each half of the lamina follows a course parallel to the edge, and gives off numerous branches, which fork repeatedly as they spread in a palmate manner towards the leaf margin. The foliage-leaves occur either scattered on long shoots of unlimited growth, or at the apex of short shoots (spurs), which may eventually elongate into long shoots. The flowers
flowers
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 flower is a shoot bearing leaves in the form of stamens. A mature pollen-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 wall
which on collapsing pro- duces a longitudinal groove similar to the median depression on the pollen-grain of a cycad. The ordinary type offemale flower has the form of a long, naked peduncle bearing a single ovule on either side of the apex (fig. 12), the base of each being enclosed by a small, collar-like rim, the nature of which has been vari- ously interpreted. A 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. developed in the upper Ex, Exine. region of the megaspore, each consisting of a large egg-cell surmounted by two neck-cells and a canal-cell which is cut off shortly before fertilization. After the entrance of the pollen-grain the pollen-chamber becomes roofed over by a blunt protuberance of nucellar tissue . The megaspore (embryo-sac) continues to grow after pollination until the greater part of the nucellus is gradually destroyed; it also gives rise to a vertical outgrowth, which projects from the apex of the megaspore as a short, thick 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
The 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 evidence afforded by normal and abnormal flowers appears to be in favour of the following 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 organs than are usually found in the recent
apex of a shoot the vascular bundles of a leaf make their Anatomy. appearance as 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 ring. Each double leaf-trace passes through four internodes before becoming a part of the 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 veins; they are abundant also in the cortex and pith of the shoot, in the fleshy integument of the ovule, and elsewhere. The secondary wood. of the shoot and root conforms in the 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 internal part of B, leaf bearing marginal ovule. (After the xylem recalls the Fujii.) cycadean type. The secondary phloem contains numerous thick-walled fibres, parenchymatous cells, and large sieve-tubes with plates on the radial walls; swollen parenchymatous cells containing crystals are commonly met with in the cortex, pith and medullary-ray tissues. The wood consists of tracheids, with circular bordered pits on their radial walls, and in the late
interest
This is not the place to discuss in detail the past history of Ginkgo (see PAL AEOBOTANY : Mesozoic). Among Palaeozoic genera there are some which bear a close resemblance to the recent
almost identical with those of the maidenhair tree, have been described from French and English localities. During the Triassic and Jurassic periods the genus Baiera--no doubt a representative of the Ginkgoaleswas widely spread throughout Europe and in other regions; Ginkgo itself occurs abundantly in Mesozoic and Tertiary rocks, and was a common plant in the Arctic regions as elsewhere during the Jurassic and Lower Cretaceous periods. Some unusually perfect Ginkgo leaves have been found in the Eocene leaf-beds between the lava-flows exposed in the cliffs of Mull (fig. II). From an evolutionary point of view, it is of interest to 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 link with both cycads and ferns. The structure of the seed, the presence of two neck-cells in the archegonia, the late
End of Article: GINKGOALES If you wish, you can link directly to this article.
<a href="http://jcsm.org/StudyCenter/Encyclopedia/GEO_GNU/GINKGOALES.html"> GINKGOALES </a> |
|
|
(Previous) GINKEL, GODART VAN (163o-17o3) |
(Next) GINSBURG, CHRISTIAN DAVID (1831 ) |
|
Sponsored Advertisements