THE ULTRASTRUCTURE OF THE SPOROPHYTE-GAMETOPHYTE JUNCTION AND ITS RELATIONSHIP TO BRYOPHYTE EVOLUTION

Abstract

  The sporophyte in bryophytes is permanently connected to the gametophyte by a basal organ, the foot, enclosed within the gametophyte vaginula. The adjoining sporophyte and gametophyte tissues form the placenta, that is the seat of active nutrient translocation. In all three major groups of bryophytes, viz. mosses, hepatics and anthocerotes, cells with wall protuberances, referred to as the transfer cells, are found in the placenta. In advanced moss taxa, transfer cells occur on both sporophytic and gametophytic side of the placenta, whereas in more primitive taxa such as Andreaea, Tetraphis and Polytrichales they are restricted to the sporophyte. Placental transfer cells are reported to be lacking in Sphagnum. In hepatics a single layer of sporophytic transfer cells and several layers of gametophytic transfer cells are found. In the anthocerotes Phaeoceros and Anthoceros transfer cells are formed only on the gametophytic side of the placenta and the sporophyte forms haustorial cells that penetrate the gametophyte tissue. Placental transfer cells differentiate very early in the moss Funaria hygrometrica, whereas they appear at rather an advanced stage of sporophyte development in the liverwort Targionia hypophylla. It is concluded that the ultrastructure and development of the placental region is a promising line of enquiry into the evolutionary relationships of bryophytes, yet successful comparison is presently hindered by the paucity of available information.