Studies of a Brittlebush population occurring in the Sonoran Desert were conducted from 1986 through 1997. One component of this work was to determine the extent to which local adaptation has occurred in the seed biology of plants growing along a short topographic/aridity gradient. Mature seeds were collected from Wash and Ridge microsites along the gradient. Current-year seed and 2-year-old seed - for the years 1994 and 1996, and 1995 and 1997 - did not exhibit adaptive differences in germination capacity or the "half-time" to reach maximum germination when comparisons were made using microsite of seed origin. Two-year storage, in paper bags at room temperature, accelerated the germination rate when seeds were provided with overhead misting. Mean germination times were near 12 days for current-year (97) seeds versus near 7 days for 2-year-old (95) seeds when temperatures during misting were about 35/25 C. At lower glasshouse temperatures, the between-year difference in germination time decreased to less than 2 days. However, dry storage always accelerated the germination rate. Other Brittlebush populations were sampled for local seed adaptations across a long distance (330 km) precipitation gradient in the Sonoran Desert. Winter rains (November 01 through May 31)for three locations were, on average: 46 mm near Yuma, AZ; 94 mm near Phoenix, AZ; and 218 mm near Bartlett Reservoir, AZ. Adaptive differences in the germination rate were evident when using seed from populations at these three sites. The shortest germination time was observed for seeds from the Bartlett Reservoir population.

Key words: Encelia farinosa , adaptation, desert, population biology, seed