Describe the modes of alternation of generation in Pteridophyte.

Ans. There is a regular feature of alternation of generations. At the time of sporogenesis the reduction division takes place and the haploid spores are formed which represent the first stage of the gametophyte generation. Later on fertilization takes place and a zygote is produced which represents the first stage of sporophyte generation. This way the gametophyte gives rise to the sporophyte by the process of syngamy and the sporophyte gives rise to the gametophyte by the process of meiosis. The gametophyte of sexual generation intervenes between meiosis and syngamy while the sporophyte or asexual generation intervenes between syngámy and meiosis. The haploid phase (x) is the gametophyte and the diploid phase (2x) is the sporophyte. These two phases follow one another in the life-cycle, in regular succession.

Under normal circumstances there is a regular alternation between a gametophyte (sexual) phase and a sporophyte (asexual) phase. The male gametes, produced in numbers from antheridia, are known as antherozoids, since they are flagellated and are able to swim in water, while the female gametes (oospheres) are non-motile and are borne singly in flask shaped archegonia. Fusion between an egg cell and an antherozoid results in the formation of a zygote, which contains the combined nuclear material of the two gametes. Its nucleus contains twice as many chromosomes as either of the gamete nuclei and it is therefore described as diploid. The zygote develops directly by mitotic divisions into the sporophyte which is likewise diploid. The spore mother cells within sporangia divide meiotically so that the nonmotile haploid spores are formed, the life-cycle is then completed when these spores germinate and grow, by mitotic division, into haploid gametophytes.

In some members of pteridophytes all spores produced by the sporangia are of one type, such a plant is known as homosporous (e.g., Lycopodium, Dryopteris etc.). On the other hand, in heterosporous pteridophytes (e.g., Selaginella, Marsilea etc.). The spores are of two kinds-the smaller spores are known as microspores or male spores and are developed in microsporangia, while the larger spores are called megaspores or female spores and are formed in megasporangia. The microspores on germination produce the male gametophytes on which, the sperms are produced. The megaspores produce the female gametophytes on which, eggs are produced.

In Bryophytes, the dominant phase in the life-cycle is the gametophyte, for the sporophyte is retained upon it through-out its life and is either partially or completely dependent on it for nutrition. Contrary to it in pteridophytes the sporophyte is the dominant generation, for it very soon becomes independent of the gametophyte (prothallus) and grows to a much greater size. The sporophytes are being differentiated into stems, leaves and roots. Only the sporophyte shows well developed conducting tissues (i.e., xylem and phloem).

The prothalli The gametophytes, in some pteridophytes are subterranean and in others they are retained within the resistant wall of the spore and are thus able to survive in a much wider range of habitats. It is notable that wherever the gametophyte is retained within the spore, the spores are of different sizes (heterosporous), the larger megaspores give rise to female prothalli which bear only archegonia, and the smaller microspores giving rise to male prothalli which bear only antheridia. The division between heterospory and homospory is one of the criteria used in the classification of pteridophytes.

The sporogonium : The sporangium, in all pteridophytes, is initiated by the laying down of cells. Since this wall is periclinal each initial cell is divided into an outer and inner daughter cell. If the sporogenous tissue is derived from the inner daughter cell, the sporangium is described as ‘eusporangiate’ and if from the outer, as ‘leptosporangiate’. In leptosporangiate forms, the sporangium wall, stalk and the spores, are derived from the outer daughter cell, but in eusporangiate forms, the adjacent cells take part in the formation of part of sporangium wall and the stalk. In eusporangiate form the sporangium is large and massive, the wall is several cells in thickness and the spore content is high. In leptosporangiate forms, the sporangium is small, the wall is one cell in thickness and the spore content is low. Of these two types, the eusporangiate is primitive and the leptosporangiate advanced.

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