The Foot of Riella americana and Its Relation to Nutrition of the Sporophyte

Abstract

1. The foot of the aquatic liverwort, Riella americana, arises from a broadening of the base of the young seta in a manner comparable with the early ontogeny of the foot of other liverworts. 2. A series of profound anatomical modifications begins when the foot is eight to ten cells long and proceeds uninterruptedly to its maturity and final degeneration. Most of the cells of the foot become enlarged into giant cells, which at its margin become gibbous and later commonly send digitate processes into the tissues of the surrounding gametophyte. The cell walls become thick and dark, the interior ones being sometimes partly resorbed so as to leave partial or overlapping walls. The nuclei are uniformly large; there are usually two and sometimes three or four in each cell. The cytoplasm becomes plasmolyzed and coarsely granular. Both cytoplasm and nucleus exhibit granular degeneration at or before the time of ripening of the spores. 3. The zone of contact between foot and gametophytic tissues is a very intimate one. 4. Since nutritional independence had been previously demonstrated for this species, the foot is considered as having, at least at maturity, the single major function of anchorage and none of absorption from the gametophyte. 5. Anatomical evidence, such as the formation of multinucleate giant cells and granular degeneration of the protoplasm, suggests the possibility of a reversal of the usually accepted nutritional relation between the two generations; the gametophyte of this species may prove to be parasitic upon the sporophyte, except in its early ontogeny.