ORIGIN OF SPOROPHYTES OF BRYOPHYTES

ORIGIN OF SPOROPHYTES OF BRYOPHYTES

            There are two theories put forward to explain the origin of sporophytes of Bryophytes: (a) the homologous (modification of transformation) theory and (b) the antithetic theory (interpolation or intercalation theory).

            1. The Homologous Theory:

            This theory was put forward by Pringsheim (1876, 1878) and restated by Scott (1896). Other advocates of this theory are Church (1919), Zimmermann (1930, 1932), Evans (1939), Fritsch (1945) and Bold (1938, 1940, 1948).

            According to this theory the spore producing structure (sporophyte) and the sex structure producing (gametophyte) generations are strictly homologous (fundamentally similar in nature) and the sporophyte is a direct modification of the gametophyte and not a new structural type. Supporters of this theory are of the opinion that the sporophyte is to be interpreted as a neutral generation and one whose function is the production of spores.

            The evidences in favour of homologous theory have been drawn from algae, bryophytes and pteridophytes. These include the isomorphic alternation found in certain algae, photosynthesis in sporophytes of bryophytes, the presence of trachieds of gametophytes of pteridophytes and the occurrence of apogamy and apospory.

            2. The Antithetic Theory:

            This theory was first formulated for the first time by Celakovsky in 1874 and was further taken up by Bower (1890, 1908, 1929, 1940), who supported it strongly from time to time, Strassburger (1894), Cavers (1910), Chamberlain (1935) and Campbell (1940).

            According to this theory the gametophytic generation is an original one and the sporophyte is an entirely new structure intercalated between two successive gametophytic generations. This theory further states that the sporophyte has been derived by the elaboration of unicellular zygote, resulting from the union of the gametes of a green algal ancestor and is interpolated in the life cycle between the successive events of fertilization and meiosis.

            According to Campbell (1940), the sporophytes of bryophytes like the zygote of algae represents the terrestrial phase of the organism compared with the aquatic or amphibious gametophytes. As the terrestrial habit becomes more and more pronounced, the sporophyte assumes increasing importance until finally it becomes the dominant phase in the life cycle.