and the pattern of variation is in some cases associated with dichogamy (Brunet 1990).
To determine whether differences in the probabilities of pollen transfer are responsible for the observed variation in sex allocation among positions, one needs to quantify the influence of dichogamy on pollen-transfer probabilities.
With synchronous flowering and dichogamy (Lloyd and Webb 1986) or temporal dioecism (Cruden and Hermann-Parker 1977), there is no overlap of pollen presentation and stigma receptivity among flowers within a plant, and hence no geitonogamous selfing.
Factors such as dichogamy
and pollinator directionality can select for specific patterns of relative male allocation among flowers (inflorescences or umbels) (Brunet and Charlesworth 1995).
To examine more precisely how dichogamy influences floral longevity, consider a protogynous flower, and let anther dehiscence begin d days (d [greater than] 0) after flower opening, that is, increasing the value of d corresponds to increasing the delay in the start of the male sexual phase relative to the start of the female sexual phase.
Exactly how optimal floral longevity is influenced by dichogamy compared with homogamy depends on the specific values of 1 - p and 1 - g.
Incidence of monecy and dichogamy in relation to self-fertilization in angiosperms.
Because field studies of the floral biology are particularly meager, data on such characteristics as dichogamy and herkogamy are very limited.
The lower rate of autogamy on the Juan Fernandez Islands may be attributable to the significant percentage of species that have temporal separation of the sexes through dichogamy (mostly protandry).
While dichogamy prevents self-pollination within a flower, self-pollination may still occur among flowers on the same plant (geitonogamy), if they are at different developmental stages.
Thus, some species (those prone to self-clogging) may evolve dichogamy even after self-incompatibility has evolved.
Elisens (1985) found that seed set from self-pollinations in species of subtribe Maurandyinae (Antirrhineae) varied at the intra-individual and interspecific levels; he characterized this subtribe as facultatively autogamous/xenogamous, with mostly showy flowers having a prevalence of dichogamy