III. On apogamy and the development of sporangia upon fren prothalli

Abstract

The two chief modifications of the normal course of the life-history of a fern, apogamy and apospory, are of interest in themselves, but have acquired a more extended importance from the possibility that their occurrence may aid in indicating the true relation between the sexual and spore-bearing generations, and so throw light on the nature of “alternation of generations” in archegoniate plants. This aspect has been recognised since the discovery of the phenomena, and will be best appreciated by tracing the progress of opinion on the nature of alternation from the time of Hofmeister to the present day. Only the more important contributions bearing on the subject can be mentioned in this place. With the publication of the ‘Vergleichende Untersuchungen’ (1851), the fact of the regular alternation of a sexual with an asexual generation in the life-history of Bryophyta, Pteridophyta, and Gymnosperms was established. Hofmeister subse­ quently extended the observation to Angiosperms. In the work mentioned, and in the ‘Higher Cryptogamia,’ published some ten years later, no views as to the nature of alternation of generations are discussed. With the extension of accurate know­ledge of the life-histories of Thallophytes, the attempt was made to compare the different individuals of the same species of Alga and Fungi with the sexual and asexual generations of archegoniate plants. Two main views of the nature of alter­nation in the latter were put forward. On the one hand, Celakovsky regarded alternation of generations in the Archegoniatse and a few Thallophyta as essentially different from that found in the majority of the latter group. He distinguished the two types as antithetic and homologous alternation respectively. Pringsheim however, held that the sexual and spore-bearing generations were homologous with one another, alike in Thallophyta and Archegoniatse. In support of this view he relied upon the instances of apospory which he had experimentally induced in Mosses, together with the occurrence of apogamy in Ferns, the first case of which had been discovered by Farlow a few years before. He also compared the life-histories of a number of Thallophytes witli one another and with that of the Moss, and showed how the reduction of the first neutral generation in some of the formerled to a condition of things not dissimilar to the relation existing between the moss sporogonium and the sexual plant. Additional cases of apogamy in Ferns were subsequently discovered by DeBary and the subject fully discussed. Subsequently Druery found the first instance of an aposporous fern, and this and other examples were investigated by Bower.