Factors Determining Fruit Set in Heterostylous Bluets, Houstonia caerulea (Rubiaceae)

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Abstract
Subpopulations of heterostylous H. caerulea consist of both pin and thrum plants (heterogeneous) or of plants of a single morph (homogeneous). Crossing experiments demonstrate that pollination between morphs is essential for maximal fruit set. In heterogeneous subpopulations, percent fruit set increases as the ratio of pins to thrums approaches unity. Fruit set also is positively correlated with subpopulation size but shows little relationship with distance to the nearest compatible subpopulation. In homogeneous subpopulations, fruit set decreases with increasing subpopulation size, and distance to the nearest compatible subpopulation is more important in determining percent fruit set. Substructuring may profoundly affect the reproductive biology of plant populations.