In the majority of the populations of seed plants, there is a single morph, a "cosex," which combines maternal and paternal functions in the same individuals. Fewer than 10% of populations are dimorphic, with differentiated males and females specializing in one sexual function. Evolution of sex conditions has been principally from cosexuality to separate sexes. Conditions under which unisexual mutants invade cosexual populations are examined. The advantages of cosexuality include the facilitation of mating, reproductive economies, noninterchangeable diminishing returns imposed on paternal and maternal fitness by Bateman's Principle and the distinct spatial distribution of maternally and paternally derived offspring, and bet-hedging. Females and males share an ability to reproduce more efficiently than cosexes under certain conditions. Females (but not males) may gain benefits from increased outcrossing, or they may gain better seed dispersal of their more massive crops, while males can conceivably obtain ...