Sunday, 3 February 2019

Ginkgoales Part 3


The ovules are pollinated soon after megaspore formation. A pollination drop is secreted at the micropyle  of  the ovule. The wind-borne pollen, after landing on the pollination drop, imbibes nutrients from the fluid, becomes heavy, sinks down the micropyle, and reaches the nucellar tip. Unpollinated ovules drop from the tree about 4 weeks after anthesis.

Post-Pollination Development  of  Male Gametophyte. On the return  of favourable weather after the winter, the pollen grains germinate in thepollen chamber (Favre-Duchartre 1956, De Sloover-Colinet  1'963),  The exine splits along the germinal furrow, the pollen tube emerges at the distal end,
and the vegetative (tube) nucleus may pass into it. The tube grows laterally and horizontally into the massive nucellus and branches profusely. The pollen tube grows in between the nucellar cells,  in  a hypha-like manner The branched pollen tube in the nucellar tissue has been studied from sections and dissections (Friedman 1987), and undoubtedly has a haustoria! function (see Chap.  2).
The proximal end  of  the pollen grain contains the prothallial, the stalk and body cells/male gametes. In the beginning it is enclosed in the exine, but later enlarges and bursts out of the exine. The short, broad end  of  the "tube" is usually unbranched  and a bunch  of  them grow into the nucellus towards the female gametophyte. Consequently, the nucellus becomes completely disorganized between the intermediary chamber and the female gametophyte. At the grain end, the pollen tubes appear to lie in a longitudinally continuous cavity in the middle  of  the nucellus.Thus, the pollen tube has: (a) a vegetative portion which grows laterally into the nucellus and is haustoria! in function, and (b) a fertile portion which carries the motile gametes. The occurrence  of  motile/ciliate sperms in  Ginkgo  was discovered by Hirase (1895). 2  This was a landmark in the history  of  (embryological) study  of  gymnoserms.

Fertilization  In zooidogamous gynmosperms, autolysis  of  the nucellus, megaspore wall, and thegametophytic tissue around the archegonia form an archegonial chamber (Pettitt 1977). The turgidnucellar cells abruptly release their vacuolar contents, a fluid is produced which forms a pool and loods the archegonial chamber and the space above it .The male gametes are released  in  this fluid(Lee 1955). The spermatozoids, with the band of flagella at their posterior end, swim about in the chamber with a forward and circular motion. The four neck cells open out  as  the egg cell pushes through the disintegrated ventral canal cell to form a beak. The egg nucleus may elongate and extend into the beak. As soon as the spermatozoid becomes attached to the projection  of  the egg, the elongated nucleus withdraws toward the centre and the beak  of  the egg retreats. According  to  Lee (1955), only the head  of  the sperm (made  of  a vacuole-like structure and cytoplasm) flows through; most  of  its body is left behind, outside the archegonium, and disorganizes immediately. However, Favre-Duchartre (1956) observed portions  of  the ciliate band  of  the sperm inside the egg. Extra sperms are prevented from entering 'into the egg cytoplasm by a thickening  of  the plasma membrane of the egg (Lee 1955). During karyogamy, the paternal chromosomes (12) become short. They can be stained clearly by Feulgen reaction  as  soon  as  the male pronucleus penetrates the female pronucleus. They mix with the maternal chromosomes (12) during the first somatic prophase (Favre-Duchartre 1958). 

The zygote nucleus divides in situ followed  by  several free-nuclear divisions. The nuclei become distributed throughout the young proembryo. Sometimes, evanescent walls appear during the free-nuclear period. During later stages, the nuclei become distributed almost evenly  in  the cytoplasm. Wall formation takes place when there are 256 free nuclei; the newly formed cells fill the entire proembryo. Within the female prothallus, the cellular proembryo develops continuously it is considerably influenced by the prevailing temperature. The cells at the base divide and function as embryonal cells, while the upper cells elongate to form a massive suspensor. There is, however, no well- defined suspensor; it is a micropylar region of elongated cells.

Differentiation of Embryo.There is a cap-like structure formed by the  upper cells (towards the neck  of  the archegonium) which  is  pierced through by the radicle at the time of germination (Favre-Duchartre 1958). The embryo develops (slowly) after the seeds have been shed. The mature embryo is dicotyledonous, occasionally three cotyledons are present. The embryo may reach its maximum size in 3 months after fertilization,  if  the development is accelerated in an incubator. In nature, the embryos may remain healthy for  12  months  if  the ovules are preserved in a damp place. This is because the embryo  is  surrounded by the prothallial cells, which provide necessary humidity for its survival (Favre-Duchartre 1958). There  is  no after-ripening requirement, and the seeds germinate whenever a suitable substrate is available. The most conspicuous change in a female gametophyte  is  the deposition of  reserve food like fat, starch and protein. Unlike other gymnosperms, the accumulation of food reserves  in  Ginkgo  occurs before fertilization  (cf.  cycads;
Favre-Duchartre 1958). 

Seed.The seed coat is contributed both by the chalaza and integument. The integument differentiates into three zones: (a) outer (parenchymatous) sarcotesta, (b) middle (sclerenchymatous) sclerotesta and (c) innermost (thinwalled) endotest Germination. a. The sarcotesta  is  5-6  mm thick in the equatorial region, and  2-3  mm in the micropylar and chalaza! region; it is the only "live" portion of the integument. The epidermis has a ca.  15-f.Lm-thick  cuticle which is interrupted above the stomata. The cells contain chloroplasts, some  of them also have druses of monohydrate calcium oxalate crystals. The bulk of the integument is formed  of  large turgid cells, the latter further enlarging towards the sclerotesta, while the number  of  chloroplasts and the size  of starch grains decrease (Favre-Duchartre 1958). The stomata comprise two kidney-shaped cells filled with starch grains; the frequency is about eight stomata per mm 2.

The mature seed is the size of a small apricot. The seed coat has  an  outer orange-coloured fleshy portion rich in butyric acid, and emits an odour like rancid butter. Inner to the fleshy layer  is  the stony layer, followed by the innermost papery layer. The dicotyledonous embryo  is  in the centre  of  the gametophyte, the so-called endosperm. The two cotyledons are normally equal and hypogeal, and have stomata mainly on the adaxial surface.

Germination.During germination, the plumule  is  pushed out of the testa by elongation and arching of the cotyledonary base. A portion of the cotyledon remains inside the seed, enlarges and persists through the first season, and functions as a haustoria! organ. The first two or three leaves on the seedling are small and scale-like. The young stem stops· its rapid elongation during the first winter after it has formed a close crown of leaves and large terminal bud.

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