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Summary Some male and female plants of dioecious populations of seven Cotula species are strictly unisexual, or constant; other plants of the same populations are inconstant, with a low proportion of florets of the opposite sex (minority-sex florets) which function normally. The distribution of minority-sex florets among heads of male and female plants is shown by rank-frequency curves. The average inconstancies of males and females are quantified as the proportions of genes transmitted via gametes of the minority-sex florets, estimated by the proportions of minority-sex florets. The average inconstancy of males is 9.9 × 10−4; that of females is 4.3 × 10−4. The Primary Sex Index, 0.9993, is the average constancy (1—inconstancy) of all individuals. This provides a measure of the degree of separation of male and female gamete functions in the two sexes, on a scale from 0.5 (sexual monomorphism) to 1.0 (complete separation of the sexes). The sex Asymmetry Index, male inconstancy–female inconstancy/male inconstancy + female inconstancy, + 0.39, measures the relative magnitude of inconstancies of the two sexes on a scale from –1.0 (androdioecy) to + 1.0 (gynodioecy). Male and female heads differ in several secondary characters. The progeny of experimental crosses show that females are heterogametic and males are homogametic for sex-determining chromosomes. Males and females occur in equal numbers in the progeny of male × female crosses and in natural populations. The distribution and experimental manipulation of sex inconstancies provide considerable information on the origin and genetic maintenance of dioecy. The evolution of dioecy in Cotula has been accomplished by divergence in the sexes of the timing of the switch from male floret to female floret formation during head development, rather than by the introduction of male- and female-sterility genes. No genes essential for female reproduction are restricted to the Y chromosome. A polyploid series in the dioecious populations (2n= 52 to 2n=c.312) indicates an ‘active Y’ mechanism, and may have been facilitated by the regeneration of sexual dimorphism from inconstant females with new chromosome numbers.

Seventeen angiosperm species were examined to determine the degree to which the plants of each sexual morph limit their maternal investment during one reproductive session at three successive floral stages (flower determination, pre-anthesis ovary development, and postanthesis fruit maturation). The results indicate that species and sexual morphs differ dramatically in the relative use they make of the three stages. Both the flower number per inflorescence and the proportion of flowers with developed ovaries are generally less variable in species with inflorescences which have well defined sequences of flower development. In most species, the variability among the plants of one morph is greater in the frequency of fruit maturation than in the frequency of ovary development. Much of the intra-morph variation in both the frequency of ovary development and the frequency of fruit maturation appears to be non-genetic. In most of the morphs examined, the frequencies of ovary development and fruit maturation are not correlated with the number of flowers per plant or with each other. The results provide circumstantial evidence in support of the hypothesis of serial adjustment of maternal investment.

Summary It is postulated that in one reproductive session the level of maternal expenditure of an angiosperm plant is determined by a temporal series of controls on the number of potential fruit in which an investment is made. The serial adjustment hypothesis has three parts: A. The amount of maternal expenditure is regulated at many developmental units, particularly single flowers and fruit, at three principal sequential stages – the determination of flowers, the development of ovaries and the maturation of fruit. B. At each stage, the initiation or continuation of an investment requires an amount of available resources above a certain threshold. Hence maternal expenditure is continually adjusted to the resources available at each developmental site. C. The pattern of controls that maximizes the maternal fitness of a plant is selected. Many factors affect the relative advantages of regulation at the three stages, therefore species and sexual morphs appear to vary widely in their proportional use of the three stages. Regulation of flower determination has the general advantages of offering bidirectional adjustment of maternal investment, maintaining a constant ratio of maternal to paternal investment, and reducing wasted expenditure. The later stages, ovary and fruit regulation, allow secondary adjustments of maternal investment in unpredictable circumstances and permit adjustment of the relative numbers of polliniferous and seminiferous flowers. The relative advantages of restricting the numbers of developing ovaries, or of maturing fruit, depend principally on the extent to which differences in the capacity of flowers to mature fruit are evident before anthesis.

Summary Four geographic races of Leavenworthia crassa and one race of L. alabamica are described as self-incompatible, since the average frequencies of fruit set and of pollen tube growth into the style are significantly lower after self-pollination than after cross-pollination. The frequency of successful self-pollination in individual plants of these races varies from zero to 100%, and is positively correlated with the average number of seeds in self-pollinated siliquas. Eleven races of L. crassa and three races of L. alabaimca are adapted to self-pollination and are described as self-compatible, although self-pollinations succeed 2% less frequently than cross-pollinations. Self-pollinated siliquas of these races contain an average of 12% fewer seeds than cross-pollinated siliquas; this is probably an expression of cryptic self-incompatibility and not the result of the exposure of seed lethals on inbreeding. There is, therefore, no absolute distinction between self-incompatibility and self-compatibility in Leavemcorthia. The growth of pollen tubes is compared in successful and unsuccessful pollinations of the same type, in self- and cross-pollinations of the same individuals, and in self-incompatible and self-compatible races. Pollen grains react with the stigma even in unsuccessful pollinations and some pollen grains are inhibited in successful pollinations. In both self-incompatible and self-compatible races, the average number of emptied pollen grains on the stigmata of successful pollinations (an estimate of the number of pollen tubes in the styles) is similar after self- and cross-pollinations. But the average number of emptied pollen grains is lower in self-incompatible races than in self-compatible races. The results are interpreted in terms of the response of pollen grains to an essential growth substance supplied by the stigma.

Summary 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.

Summary Five classes of sex expression are distinguished in Cotula on the basis of the degree of separation of male and female gametes on different flowers and plants. These classes are divided into sexually monomorphic and dimorphic systems. The order–monocliny, gynomonoecy, monoecy, subdioecy, dioecy–places the five classes in a linear sequence of increasing dependence for fertilization on the transfer of pollen between flowers and plants. Four of the five major taxonomic groups in Cotula and five species have more than one class of sex expression. The four monoecious and gynomonoecious species tested are self-compatible. The morphology of hermaphrodite, male and female florets are compared. Seven additional characters are associated with sex expression in New Zealand species. In general, the extent of protogyny, brightness of flowers, degree of retraction of stigmas and anthers, floret length, head diameter, peduncle length and the number of pollen grains increase from gynomonoecious through monoecious to dimorphic (dioecious or subdioecious) populations. But within each kind of sex expression there is considerable variation in these ancillary characters. A MAS (measurements associated with sex expression) index is calculated from the four quantitative characters for each New Zealand species (or part-species where there are two kinds of sex expression in a species). The MAS index reflects the expenditure of energy on adaptations to cross-pollination and provides an approximate measure of the relative frequency of outbreeding. The MAS indices of the species and part-species indicate a continuous array in the expression of adaptations to cross-pollination and the frequency of outbreeding. The different kinds of sex expression occupy characteristic but overlapping segments of this array and ought to be considered as convenient classes which only specify the breeding system approximately.