Describe with neat labelled diagram of carposporophyte of Polysiphonia.

Ans. After fertilization, certain changes in reproductive structure are being accounted in order to form the Rhodophycean fructification as cystocarp which represents the carposporophyte meant for producing the diploid carposporangia, the progenitors of carpospores.

Soon after the formation of zygote, an auxiliary cell lying adjacent to carpogonium develops from the upper end of the supporting cell; this very cell disorganises its own haploid nucleus.

An outgrowth from each of the carpogonium and auxiliary cell arises in opposite direction at the same spot. Both the outgrowths grow and meet with each other; their intervening wall dissolves and establishes a relation between them. This operation is immediately followed by the migration of zygote nucleus from carpogonium to auxiliary cell. At this stage the auxiliary cell, having a diploid zygote nucleus, is treated to be a progenitor of cystocarp in general and gonimoblast filaments in particular.

The carpogonial filament now starts to shrivel, and the enveloping threads of gametophytic origin start to grow around the developing carposporophyte.

The zygote nucleus divides into two nuclei, one of these migrates into a bud arising from the upper end of auxiliary cell. This very bud, being delimited by a cross wall at its base, is called gonimoblast initial cell which proliferates as several short laterals-the gonimoblast filaments. The topmost cell of these filaments gets enlarged as a carposporangium being clavate or pear-shaped.

With the formation of gonimoblast filaments, the supporting cell, basal cell and the topmost axial cell just below the basal cell start fusing to form an amorphous placental element which is meant to nourish the carposporophyte. At this stage, all the gonimoblast filaments compacted together seem to originate from the placental element.

The developing envelope threads now encircle the carposporophyte as a pericarp or the wall of cystocarp, being 1-celled-thick. Fully-developed cystocarp now is an urceolate ostiolate structure being cytologically diploid except for pericarp which is of gametophytic origin.

Cystocarp = Auxiliary cell + Gonimoblast filaments

(2X)                        (X)

+ Carposporangia + Pericarp

(2X)                      (X)

Carposporophyte represents the primary sporophytic phase of reduced nature being totally dependent on the gametophyte. It reproduces asexually by means of diploid carpospore produced by the carposporangia residing in the urceolate cystocarp.

Formation, liberation and germination of carpospore: The entire protoplast of a sporangium gets transformed into a clavate diploid carpospore which liberates through the ostiole of the carposporophyte.

On liberation, the carpospore secretes a wall layer, and comes to rest by facing its tapering end in contact with the aquatic object. During germination, the first transverse division produces two unequal cells i.e., the smaller basal cell will form the rhizoidal structure, while the bigger upper cell, owing to repeated cell division, is destined to form the heterotrichous filaments of the tetrasporophyte.

Tetrasporophyte: It is quite independent of the female gametophyte and represents a secondary sporophytic phase. Its external and internal morphologies are quite similar to gametophytic plants. It reproduces asexually by meio-tetraspores produced inside the tetrasporangia.

Tetrasporangia : Several tetrasporangia, being diploid spherical structures, are borne in uniseriate order at the dichotomy of the erect polysiphonous filaments.

These which originate due to proliferation of axial siphonous cells and behaving as tetrasporangial mother cells, lie in between axial and pericentral siphons, the latter constitute the cover cells covering the sporangia completely.

Formation, liberation and germination of tetraspores: The entire protoplast of a sporangium, on account of meiotic divisions, gets transformed into a tetrahedral-tetrad of four meio-tetraspores being haploid (Fig.)

Meiospores, on separation from the tetrad, become spherical. On maturity of tetraspores, the sporangial wall dehisces, and two cover cells spread apart longitudinally, and thus they liberate.

The liberated non-motile haploid (X) tetraspores in contact with the ideal aquatic substrate start to germinate in the same way as exhibited by carpospores (Fig. G-H). On successful germination of tetraspores, the sexual segregation in the gametophytic plant occurs in equal proportion-i.e., 50% male: 50% female.

Formation In Rhodophyceae Following the fertilization of egg. appreciable changes in the female sex organ and its associated structures are being accounted in order to form the rhodophycean fructification as a cystocarp. The cystocarp represents the carposporophyte, the primary haploid or diploid asexual stage in the life-cycle of red algae. This sporophytic phase, which is totally dependent on the gametophyte (but strictly on the female gametophyte in dioecious form), reproduces asexually by means of carpospores produced inside the carposporangia.

Events of cystocarp or carpospore formation : All these are the different aspects of the same phenomenon, since the events responsible for the formation of cystocarp or carpospores involve post-fertilization changes.

After the formation of zygote, an auxiliary cell adjacent to the carpogonium develops from the upper end of the supporting cell. This is followed by the development of an outgrowth from each of the carpogonium and auxiliary cell in opposite directions exactly at the same spot; these. outgrowths meet each other. The intervening wall dissolves, forming a passage through which the zygote nucleus migrates from carpogonium to auxiliary cell, while the latter withdraws its own haploid nucleus of gametophytic origin. The auxiliary cell, having thus a diploid nucleus, is treated to be a progenitor of cystocarp.

The carpogonial filament at this stage starts to wither, and the enveloping threads around the reproductive and its associated structure continue to grow.

The zygote nucleus divides freely into two nuclei, one of these migrates into an outgrowth arising from the upper end of the auxiliary cell. This unnucleate outgrowth behaves as a gonimoblast initial by developing a cross wall at its base. This initial cell now starts to proliferate as several short laterals, the gonimoblast filaments. The terminal cell of each of the filament gets enlarged as a carposporangium being clavate or pear-shaped.

With the development of gonimoblast filaments, the supporting cell, basal cell and a part of axial siphonous cell just below the basal cell start fusing to form an amorphous placental element. So, at this stage, all the gonimoblast filaments compacted together seem to originate from the placental element.

The developing envelope threads now encircle the fructification as a pericarp or the wall of cystocarp being one-celled thick having a distinct pore at the tip.

The entire protoplast of a sporangium gets metamorphosed as a clavate diploid carpospore. The entire events of post-fertilization changes responsible for producing the carposporophyte, thus, are fulfilled.

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