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As clonal plants get larger, individual flowers become more surrounded by other flowers borne on the same clone. With very restricted movement of pollen reported for both animal- and wind-pollinated species, the breeding system may change as a consequence of a clonal plant covering greater amounts of space. In Carex platyphylla, a wind-pollinated, self-compatible sedge, the absolute and relative amount of endogenous pollen per flowering culm increases sharply as a plant grows from one to about 10 culms, then increases at a slower rate, given a constant rain of exogenous pollen. In Trifolium repens, a bee-pollinated, self-incompatible, stoloniferous clover, a model based on known patterns of flower placement and stolon growth patterns, coupled with estimates of pollen-deposition and floret-visitation patterns by bees, suggests that the number and length of stolon internodes affect successful pollination. Also, the distribution of seeds among inflorescences is affected by clonal growth patterns and bee behavior at each floret. The dynamics of internode initiation and growth also change the rate of interdigitation of clones, which will subsequently affect the pollination ecology. An additional, field test with the viny herb Cucumis sativus demonstrated that as plants increase in size and in number of flowers, the pollinator pool, the success of flowers in setting seed, and gene flow distance all change. Shifts in breeding systems as a function of plant growth and flower placement may be widespread among clonal and nonclonal plants. This relationship is very sensitive to short distance dispersal of pollen, the behavior and diversity of pollinators, and the community structure around the population. Internal controls on fruit set within the clone and evolutionary responses of the plant (altered phenology, genetic changes in the breeding system) may interact with the geometry of clonal spread in determining gene flow patterns. Change in mating patterns may be common, and species and populations cannot be characterized without consideration of their current population structure and ecology.