Sex determination is a process that leads to the physical separation of male and female gamete-producing structures to different individuals of a species. Sexual reproduction is an ancient feature of eukaryote life, yet the sexes as we currently recognize them are relative late comers in the evolution of sex. Sex determination systems in plants have evolved many times from hermaphroditic ancestors (including monoecious plants with separate male and female flowers on the same individual),and sex chromosome systems have arisen several times in flowering plant evolution. Sex chromosome evolution is intimately connected with Y chromosome degeneration. Most current understanding of how the distinctive properties of Y chromosomes evolved comes from theoretical work on the evolution of genomic regions with low recombination. The identification of sex chromosomes in plants is problematic because most of them do not differ morphologically from autosomes or from one another. For example in some species, such as Actinidia deliciosa var. deliciosa, X and Y chromosomes are too small to support observations of their distinguishing characteristics.) In the majority of plants, male and female organs are formed and developed simultaneously, but only up to a point when the growth of either set of sex organs is inhibited. Inhibition of sexual development can vary in character so that in most cases, sexual development is inhibited by the absence of cell division. In many species of bryophytes, heterothallism (unisexuality) has been correlated with the presence of sex chromosomes. Although the extent of heterothallism and sex chromosomes in the bryophytes has not been assessed systematically, this is the only known group of homosporous plants that uses sex chromosomes in sex determination. To date, studies of bryophyte sex determination have focused on the heterothallic liverwort Marchantia polymorpha. Many dioecious species, including those with well-developed sex chromosomes, ........