The distribution of male and female florets in heads of monoecious populations is represented by a curve joining points on a graph of percentage of female florets against rank in the same percentage. Comparison of curves for samples from different origins in Cotula minor and C. atrata subsp. atrata shows that sex expression is not greatly influenced by macro-environmental differences. Much of the total intra-population variation in sex expression appears in the heads collected at one time from a single plant. The rank-frequency curves show marked differences between species in the percentages of male and female florets. The average percentage of female florets in seventeen monoecious species is negatively correlated with the floret length, a measure of adaptation to outbreeding. In three species, the seed set and subsequent germination percentages are slightly lower after self-pollination than after cross-pollination. Self-fertilization probably occurs in varying degrees in the monoecious species. Monoecy has arisen independently in sections Leptinella and Strongylosperma.