Sunday, 3 February 2019

Ginkgoales Part 2



Root. The young roots are usually diarch. The endodermis has conspicuous thickenings on its radial walls and there is a broad pericycle. Older roots may be tetrach or hexarch. A radiallongisection shows that the spiral elements of  the protoxylem are followed successively by traceheids with (a) reticulate pitting,  (b)  transversely elongated simple pits and (c) bordered pits.

Secondary growth occurs, but annual rings are not pronounced. The tracheids have thinner walls. Some xylem parenchyma cells include crystals, and thick-walled fibres are abundant in phloem. The medullary rays are one to several cells high and often show crystals.

Stem. The shoot apex consists  of  superficial apical initials, a prominent subapical zone of central mother cells, and a zone  of  rib meristem around it  on  its proximal side (Foster 1938).The secondary phloem is composed of sieve elements, parenchyma strands and fibres  in  the axial system and rays  in  the radial system (Srivastava 1963a). The albuminous cells associated with the sieve cells lack starch and are crushed in the old phloem. They are connected to the sieve cells by one-sided sieve areas. Lateral connections between the sieve elements and albuminous cells could not be traced,  as  callose was not present in sufficient quantity. These cells also have plastids but  do  not store normal, detectable starch. The fibers are elongated, tapering, tangentially flattened and non-septate. They have narrow lumen, the wall is thick, lamellated and appears to be cellulosic. They  do  not stain positive with phloroglucinol and HCl and are strongly birefringent under polarized light '(see Paliwal 1992).

Outside the phloem there is a ring of sclereids, probably pericyclic, and an inner ring of thickened cells, whose walls appear to be gelatinized. Throughout the plant numerous mucilage canals occur  in  the pith and the cortex.

Leaf. The leaf has a double trace. A transection  of  the petiole shows two endarch vascular bundles. The primary xylem of the stem branches sympodially when the leaf traces are given off. The two traces to any leaf therefore arise  i n d ~ p e n d e n t l y  from two different primary strands. They divide at the base  of  the blade and the resultant four strands fork repeatedly to form the dichotomous system  of  veins which occasionally anastomose in the lamina (see Stewart 1983). The venation of each  of  the two halves  of the leaf is completely independent. Mucilage canals are present even between the veins  of  the leaf.

On the lower epidermis, stomata occur irregularly scattered between the veins. They are haplocheilic, surrounded by four to six ·subsidiary cells, each with a blunt papilla which projects over the guard cells (see Stewart 1983). The characteristic accessory cells of the stomata are also recognizable in the extinct taxa.

Ginkgo  is dioecious. 1  The male cones are pendant and catkin-like, borne on short shoots in the axil of normal leaves or scale leaves. The ovulate cones are borne in groups at the apex  of  the dwarf shoot. They are reduced, and each shoot bears two ovules on a long peduncle  in  the axil of a scale leaf.

Male Cone
A male cone comprises  40-50  microsporophylls . Each microsporophyll has a terminal knob, which contains a mucilage sac and there are two (occasionally three to seven) pendulous microsporangia which dehisce by longitudinal slits.

Microsporangium. The strobili are initiated in summer, and appear as small papilae  in  the axils  of  bracts. Wolniak (1976) examined more than 300  cones, and observed: (a) There is no acropetal or basipetal progression in sporangia! development; (b) There is  no  correlation between the size  of the sporangium and its development.

 ln  the  Botanical garden at lnsbuck (Austria), on a female tree  of  Ginkgo,  a branch from a male tree was grafted. The male cones developed and produced fertile pollen grains, pollination and fertilization occurred normally, and numerous (apricot-coloured) ripe seeds developed  on  the female tree (BMJ, pers. observ. 1957).development, and  in  a sporangia! pair the rnicrosporocytes are ontogenetically similar. Earlier stages have not been observed but there is evidence  of  a single hypodermal cell which divides by anticlinal and periclinal divisions. The outer cells form a wall  of  four to seven layers, and the inner cells give rise to a large group  of  sporogenous cells. A tapetum surrounds the sporogenous tissue, and a peritapetal membrane has been reported (Pettitt 1966).

An  endothecium  differentiates,  which  is an  exception,  since  in gymnosperms only an exothecium is reported. The endothecium develops from one to three layers  of  subepidermal cells, becomes thick-walled and develops fibrous thickenings.

Microsporogenesis.  Meiosis in microspore mother cells coincides with the opening  of  the bud scales of the spur shoot. The distribution  of  starch in the microsporangium is specific, and first appears at the archesporia! stage. Prior to meiosis, starch reappears in the sporogenous cells and then in tapetal cells. During meiosis I starch grains accumulate at the equatorial plate  at  metaphase, move to the opposite poles during anaphase and telophase,
and become equally distributed after the completion  of  meiosis. After telophase I, besides starch, entire plastids and mitochondria shift to the equatorial region  of  the microspore until nuclear divisions cease. Microtubules and ER proliferations appear  to  stabilize.the organelle distribution through meiosis  II.  Tetrads are formed by centripetal wall formation, and the organelles become distributed equally (Wolniak 1976). A callose wall has been observed during microspore mitosis (Gorska-Brylass 1968).

Male Gametophyte. The rnicrospore nucleus cuts off two prothallial cells the first cell towards the wall is ephemeral while the second persists. The antheridial initial divides and forms a smaller antheridial cell, which remains attached  to  the intine, and a larger tube cell, which becomes vacuolate and has a conspicuous nucleus. The antheridial cell divides periclinally to form the stalk cell (toward the pollen wall) with a distinct wall, and the body cell  The stalk and body cells persist in situ. The persistent prothallial cell remains active and
grows into the stalk cell which lies next to it. The stalk cell thus appears to form a jacket around the protruding prothallial cell . The microsporangium dehisces by a longitudinal slit along the inner face. The pollen is shed at the four-celled stage: two prothallials, one antheridial and a tube cell.

Ovule The peduncle bifurcates and bears on each branch a single sessile ovule with a fleshy collar round its base.The morphology of the collar has been variously interpreted (see  hamberlain 1935); it does not grow after pollination. Usually there are only two ovules on each peduncle,occasionally three, four or more. Whatever the number  of  ovules, the peduncle always s twice the number  of  vascular bundles. The morphology  of  the meristem which gives rise to  the ovule needs a critical reinvestigation. The ovule is orthotropous with a beakednucellus which has a heavily cutinizedpidermis. The nucellus  has  a well-differentiated strand  of  elongated cells and extends almost  to  its entire length.Its degeneration forms a narrow, deep pollen chamber (De Sloover-Colinet 1963). The inner cells degenerate first followed by the epidermis. There is a single integument, which is free from the nucellus at the apex. Two unbranched vascular strands supply the base  of  the integument.

Megasporogenesis. One or more megaspore mother celVs become distinct by their prominent nuclei and dense cytoplasm.  Due  to a considerable thickening  of  the middle lamella (Fig. 12.8 A), and the development  of  a double-layered wall, the wall  of  the mother cell becomes thick and two layered (Stewart and Gifford 1967).  The  latter has densely staining outer layer which resembles the circumjacent nutritive tissue, and an inner layer which is similar to the middle lamella except for a tighter arrangement  of the fibrillar structure.  The  young megaspore mother cell is spherical, and has a large nucleus in the centre. Its cytoplasm occasionally shows a small vacuole, relatively scanty endoplasmic reticulum (Stewart and Gifford 1967), and randomly placed starch-bearing plastids, mitochondria and dictyosomes. The mother cell elongates as it matures. The cytoplasm in the micropylar half shows a large and complex system  of  ER  while it is relatively meagre and there is no definite pattern in other parts  of  the cell. However, all over the cell, ER  is  only sparsely associated with ribosomes. In the mature mother cell the micropylar ER becomes reticulated with several "loops" and "circles" . A vacuole appears below the nucleus  of  a mature mother cell, and several small vacuoles in the micropylar part. From the micropylar half, the plastids and mitochondria shift laterally to  the chalazal end  of  the maturing mother cell the micropylar ER  may have a role in the migration  of  these organelles. In a mature cell the plastids with a few mitochondria are restricted to the region below thenucleus, the mitochondria lie just below the plastids. Other organelles, like dictyosomes, lipid droplets, dense bodies bounded by unit membrane, and  an  occasional multivesicular body,  do  not show polarity in their distribution.
Female Gametophyte. The female gametophyte develops from the haploid chalaza) megaspore. The gametophytic tissue, however,  is  not uniformly haploid, as shown by cytological studies and cytophotometric measurements of  the DNA content (A vanzi and Cionini  1971  ). At the beginning  of  the cellular stage, 5% nuclei of the gametophyte show  1C  DNA content, 50% 2C, more than 40% 4C, and the remaining nuclei 8C or higher DNA content. This variation in the DNA content is attributed to endopolyploidy. The cells with 4C content are mostly located in the outer region  of  the gametophyte. The nuclei with higher DNA content (8C or more) degenerate in the young gametophyte. In older stages, most cells contain 2C DNA.

Free-nuclear divisions  occur in the megaspore for about 4 weeks. According to Favre-Duchartre (1958), there are  13  successive mitotic cycles, so that more than 8000 nuclei are formed. The divisions are initiated at the chalaza! end, and proceed towards the micropyle.  The prothallial cytoplasm, throughout cenocytic phase, adheres to the megaspore membrane. Walls are laid down at the end  of  the 14th mitotic wave. The gametophyte remains colourless throughout the free-nuclear stage. Typical alveoli are formed, followed by cellularization. The gametophyte becomes green (due to the presence  of  chlorophyll) and starch is synthesized. The female gametophyte  of  Ginkgo biloba  is the only seed plant with a chlorophyllous gametophyte. The relative transluscence of  the integumentary tissues  of  the ovule permits sufficient light to induce the synthesis  of  chlorophyll (Friedman and Goliber 1986). The plastids do not contain an organized thylakoid membrane system (Pettitt 1977).When cell formation begins, the female gametophyte has a light green colour, attributed to chlorophyll. EM  of  chloroplasts demonstrate stacking  of  thylakoid membrane in the grana. Plastids located deeper in the gametophyte have fewer thylakoid membranes and may also show prolamellar bodies.
The presence  of  chlorophyll has been confirmed by Burgerstein (1900). He  showed that an alcoholic extract  of  the gametophyte fluoresces red; Carothers  (1907) suggested it may be  capable  of  photosynthesis. A measurement  of  photosynthetic active radiation (PAR) indicates that a gametophyte (growing within an ovule) can receive significant quantities of  light, i.e 70  f.1  mol photons m·  2  s·  1  (Friedman and Goliber 1986). This unique ability to produce chlorophyll and perform photosynthesis results from its·exposure  to  sufficient levels  of  light, and an inclination to react to this stimulus by the development  of  functional photosynthetic apparatus. The  entire gametophytes were dissected free from the ovules. They were capable  of  gross photosynthesis (not net photosynthesis) under experimental conditions. On a dry weight basis, the maximum rate  of  carbon fixation, in near-saturating light intensities, was 3.64 x  10· 3  ).1  mol  C02  g·'s·'  (Friedman and Goliber 1986).
The egg cytoplasm has the usual organelles like ER  plastids, Golgi bodies, ribosomes and a large number  of  mitochondria. Observations with an electron microscope (Camefort 1965a) have revealed the following ·cytoplasmic formations (so-called proteid vacuoles): (a) The morphology  of  small inclusions is somewhat different from others. They are completely enclosed in the double membrane  of  ER whose components stay together. An enveloping vacuole is thus absent. (b) Microbodies are present in abundance, have dense contents enclosed by a single membrane of ER. Their morphology is similar to certain lysosomes in animal cells. (c) Vesicular bodies occur seldom and comprise a mass  of  vescicles enclosed by a single membrane. The amyloplasts in a mature egg are distributed at the periphery. They are enclosed in a layer  of  endoplasmic reticulum, in addition to their own membranes (Camefort 1965a). The amyloplasts continue to fragment until the egg is mature.

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