THE ORIGIN OF THE LAND PLANT SPOROPHYTE: AN INTERPOLATIONAL SCENARIO

The origin of terrestrial plants from a charophycean ancestor is assumed as a basis for the consideration of the origin of life histories amongst this group. Charophycean algae are vegetatively gametophytic, thus requiring the interpolation of the multicellular sporophytic stage. The model presented here derives the sporophyte and typical sporangial contents from the zygospore produced from the reproductive structure of a hypothetical charophycean land plant ancestor. The presence of monads, dyads, and various forms of tetrads in the fossil record of presumed land plants add support to this model. In addition, such spore types suggest that the temporal relationship of meiosis to sporopollenin deposition was less strictly controlled than in extant land plants.