THE GAMETOPHYTE GENERATION OF SOME EARLY DEVONIAN LAND PLANTS

We summarize the current information on gametophytic plants from the Lower Devonian that are represented by three types based on permineralizations in the Rhynie Chert, two of which are antheridial (Lyonophyton and Kidstonophyton) and one that is archegoniate (Langiophyton). Impression and compression remains attributed to Sciadophyton and Calyculiphyton also are interpreted as gametophytic, based on their similarity to the permineralized forms. All are axial structures, with some axes terminating in cup-shaped gametangiophores on or in which gametangia occur, and all exhibit preserved cuticles with stomata and conducting tissue. Similarity in epidermal and conducting cell characters serves as a basis for postulating sporophyte relationships for the Rhynie Chert gametophytes as follows (N-2N): Lyonophyton-Aglaophyton, Kidstonophyton-Nothia, and Langiophyton-Horneophyton. Comparison with bryophytes and tracheophytes shows that these forms exhibit unique combinations of characters. They are more advanced than bryophytes in some features but differ in conducting cell type from tracheophytes. Thus they may represent lineages distinct from either of these groups. Their construction indicates some degree of morphological similarity between haploid and diploid thalli in the life cycles of these entities, thus supporting to some extent an alternation of isomorphic generations and the homologous theory of sporophyte origin for land plants. Alternatively, these data may indicate the early establishment of axial gametophytic structures within the context of an antithetic theory of sporophyte origin.