THE EARLY EVOLUTION OF THE ARGHEGONIATAE: A RE-APPRAISAL

Abstract

After a re-appraisal of the alternative hypotheses concerning the origin and the early evolution of the archegoniate land plants, the postulation of a thalassiophytic group of precursors with free isomorphic alternating generations by Church, Zimmermann, and several others is rej̈ected. Several versions and modifications of this ‘homologous’ theory, such as Fritsch's suggestion of diminutive filamentous algal being the progenitors of the vascular plants and Jeffrey's idea of an advent of partly emerged (aerial) sporangiate extensions of an aquatic sporophyte, are equally untenable. In the present author's opinion the old and reputedly almost obsolete ‘antithetic’ theory is much more compatible with the relevant evidence, provided this hypothesis is more precisely formulated. The conquest of the land must have begun with the gradual migration of the gametophytic (haploid) generation to semi-terrestrial or temporarily dry habitats before the development of a vegetative diploid phase. The first stage after the colonisation of semi-terrestrial and subsequently of terrestrial environments by presumably still prostrate life forms consisting of filamentous algal strands must have been the differentiation of a portion of this haploid type into a more massive parenchymatous soma which remained prostrate in many cases and bore the sessile sporangia. In bryophytes such differentiations of the gametophyte may themselves be more or less erect and thus raise the sporangia into an undeniably aerial position (as in many Musci), or they produce upright extensions on which the gametangia are produced and, later on, the sporangia are inserted (as in Marchantiales). Ontogenetically the phylogenetic sequence is frequently recapitulated in that first a filamentous protonema develops from a germinated spore and only afterwards a more important gametophytic structure (typically developed in the Bryales). In other instances the sporangium is, or was, not supported and ‘lifted’ into the air by a part of the haploid plant but by an intercalated sporangial stalk (sporangiophore) which is a vegetative diploid structure and constitutes the modest beginning of the large sporophytes of the Higher Cormophyta. The intercalated vegetative diploid structure originated subaerially and was ab initio a terrestrial organism, i.e., the sporophyte did not develop out of a submerse aquatic archetype by a process of adaptation as is assumed in the ‘homologous’ theory. The vegetative sporophyte and, accordingly, the characteristic features of the terrestrial plants such as the vascular (stelic) tissues, on the contrary developed de novo and the sporangiophore is consequently sui generis in respect of both the gametophytic and the sporangium (representing the phylogenetically older portions of the plant). The Anthocerotales are reminiscent of this early evolutionary level of the Hemitracheophyta which must ab initio have had, or soon acquired, green and partly independent (photo-autotrophic) sporophytes. The phylogenetic history of the Bryophyta and of the Hemitrachyophyta is discussed in the light of the new interpretation of the subaerial transmigration Certain pertinent problems of evolution (mono- or polyrheithry) and of homology (morphological equivalence of certain structures versus inhomology through independent, i.e., sui generis, origin) are tentatively analysed. These considerations have an important bearing on our views concerning the relationships of various archegoniate groups and reveal flaws in several ‘established” ideas some of which concern certain aspects of the Telome Theory.