Can classifications of functional gender be extended to all land plants?

Land plants (bryophytes, seedless vascular plants and seed plants) may have combined or separate sexes, and this variation also may occur at two life-cycle stages. Thus plants show variation in individuals’ attainment of fitness via sperms versus eggs (functional gender) and the diversity of gender morphs found in populations. We extend D.G. Lloyd's classification of flowering plant gender to all land plants, with three main functional classes according to whether populations are dimorphic or monomorphic for gender (i.e., populations consist of either one or two distinct sex classes), and at which life cycle stages this occurs: (1) sporophyte-dimorphic, (2) sporophyte-cosexual and gametophyte-dimorphic, and (3) gametophyte-cosexual. In dimorphic sporophytes and gametophytes, morphs that reproduce mostly as females and males may be constant (dioecy) or inconstant (gynodioecy, androdioecy, trioecy). We suggest that examining the sex conditions of seedless plants using a functional perspective will reveal a diversity of sexual systems largely analogous to those found in seed plants. An extended suite of model plants with different biological attributes will allow new tests of existing models of mechanisms that select for different sexual systems, and may lead to important new questions in the field, some of which we suggest here.