The fitness consequences of self‐fertilization are largely determined by how self‐pollination occurs. Within‐flower self‐pollination (autogamy) may be advantageous, since it can provide reproductive assurance without much seed or pollen discounting. In contrast, between‐flower self‐pollination (geitonogamy) provides no reproductive assurance and can cause severe seed and pollen discounting. I used floral emasculations with marker‐gene analysis to estimate the components of self‐fertilization in a tristylous, self‐compatible, clonal, mass‐flowering plant, Decodon verticillatus. This species produces 30% of progeny through selfing. I assessed the contribution of autogamy to selfing by comparing pollen deposition, seed production, and the selfing rate of flowers emasculated before anther dehiscence with intact flowers. Emasculation had no effect on pollen deposition, caused a small increase in seed production, and only reduced self‐fertilization by 14%, suggesting that most selfing occurs through geitonogamy. Geitonogamy can be due to self‐pollination between flowers on the same branch, different flowering branches of the same plant, or different ramets of the same clonal genet. I assessed the contribution of within‐branch geitonogamy by emasculating all flowers on a branch, which reduced selfing from 0.29 ± 0.03 (mean ± 1 se) to 0.19 ± 0.04. Between‐branch geitonogamy was estimated by comparing the selfing rate of plants with only a single flowering branch (0.16 ± 0.07) to plants with multiple flowering branches (0.27 ± 0.04). Selfing rates of individual branches also correlated positively with the daily number of flowers open on nonfocal branches of the same plant. Between‐ramet geitonogamy was suggested by significant self‐fertilization by single‐branch plants, even when all flowers were emasculated (0.10 ± 0.07). Selfing rates of individual branches also correlated negatively with measures of local clonal diversity. Based on these results, autogamy accounts for only 18 ± 14% of self‐fertilization, with the remainder (82 ± 17%) due to geitonogamy, which occurs about equally through pollination within branches (31 ± 22%), between branches (38 ± 32%), and between ramets (31 ± 28%). Because selfing occurs mostly through geitonogamy and is associated with strong inbreeding depression, it seems disadvantageous. Selfing in D. verticillatus has probably not been selected directly, but is a by‐product of self‐compatibility, large plant size, mass‐flowering, and clonal propagation.
Placed a flowering branch on each plant, emasculated one flower in each pair, collected flowers