Relationship of Plant Size to Self‐Pollination in Mertensia Ciliata

This study tested the hypothesis that self—pollination and, as a result, inbreeding rate increased with plant size in Mertensia ciliata, a self—compatible, profusely flowering perennial. The hypothesis is based on two subsidiary hypotheses, which were examined: (1) the dispersal of pollen among flowers and among plants is generally limited in plant populations; and (2) individual pollinators are likely to make longer visits to large than to small plants, and to spread higher proportions of self pollen rather than foreign pollen among flowers of large plants. Because M. ciliata exhibits inbreeding depression in seed—set and seed mass, increased self—pollination and inbreeding would lower the reproductive output of flowers on large plants. However, in experiments with bumble bees confined to nylon mesh cages, I found that the carry—over of pollen between flowers by bees is extensive in M. ciliata. In addition, individual bees visit only a few more flowers per visit to large than to small plants, and large plants attract more bees per minutes than small plants. Using these data, I constructed a mathematical model to estimate the amount of foreign pollen a flower should receive as a function of plant size. The model predicts that flowers on all plants should receive approximately equal amounts of foreign pollen, and that self—pollination and hence inbreeding will not vary with plant size. These predictions were supported by direct measures of flowers in the field: no differences were found in foreign pollen receipt and in reproductive output (seed—set and seed mass) between flowers from large and small plants.