Pterostegia drymarioides Fischer & Meyer is a monospecific taxon in the Polygonaceae (subfamily Eriogonoideae; tribe Pterostegieae). First described from horticultural specimens in St. Petersburg (Leningrad), Russia, it is a monoecious, sprawling, wirey annual with fan-shaped leaves resembling those of Drymaria (Caryophyllaceae), hence the specific epithet. The range of Pterostegia extends from Baja California, Mexico northward through California, including off-shore islands, and eastward into parts of Nevada, Arizona, and Utah. Traditionally, the fruit of Pterostegia has been characterized as an achene loosely invested by an involucral bract that is highly modified into a somewhat gibbous winged structure. Examination of plants in the field in southern California, and subsequently, herbarium specimens from throughout the range of the taxon, reveal the presence of two distinct fruit types produced on the same plant: the typical winged fruit, and a nutlet-like fruit in which the involucral bract is not inflated and closely invests the mature achene. To our knowledge, fruit dimorphism has not been reported previously for Pterostegia. Our study characterized the variation in fruit morphology using SEM. We hypothesized that the winged fruit are more readily dispersed by wind than the nutlet-type fruit. This was tested experimentally and notable differences in distance dispersed were recorded. Common garden trials were conducted to test for differences in germination rate between winged and nutlet fruit.

Key words: dispersal, fruit dimorphism, Pterostegia, SEM