MOORE, LAURA A.*, RASHMI PANT, AND STEVE BOYD. Herbarium, Rancho Santa Ana Botanic Garden, Claremont, CA 91711. - Fruit dimorphism and dispersal in Pterostegia drymarioides.
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