Search

Stamens of Berberis thunbergii respond to a tactile stimulus by snapping toward the stigma. When filaments are stimulated ('tripped'), anthers that strike an object (e.g., a glass cover slip or floral visitor) deposit some of the sticky pollen that is held together by viscin threads. When stamens were stimulated experimentally, 53% of the total pollen was deposited on the first trip, and 20% on the second, resulting in a pattern of diminishing proportional removal. Pollen removal by bees resulted in a similar pattern. However, small bees (e.g., Andrenids) tripped fewer stamens per visit than large bees (i.e., Bombus spp.). Our results do not support either the 'numerical' or 'proportional' removal patterns of Harder and Thomson (1989) but do conform to the diminishing proportional removal model proposed by Harder (1990) for plant/pollinator associations where there is a high variance in the number of pollinator visits.

Size-related patterns of floral sex allocation were examined in 14 anemophilous and eight entomophilous monoecious plants. Among the anemophilous species, eight increased male floral allocation and six increased female floral allocation with increased plant size. An increase in female floral allocation with increased plant size occurred in all eight entomophilous monoecious species examined. Pollination vector has a significant effect on size-related floral sex ratio patterns. We present a heuristic model that considers pollen dispersal of anemophilous monoecious plants as a function of plant geometry. The surface area to volume ratio of plants approximating geometric extremes (cylindrical or spherical forms) should affect pollen dispersal and floral sex ratio due to aerodynamic effects on the whole plant. Plants approximating cylinders should increase male floral allocation with increasing height. Plants that approximate a more spherical form should increase in female floral allocation with increasing size. All entomophilous plants conformed to a resource limitation model; however, anemophilous plants increased in either male or female floral allocation with increased size. Results for 13 anemophilous species support the predictions of our geometric model. One species does not approximate an extreme geometry and does not appear to conform to the geometrical model's predictions.

Conditions just prior to field abandonment and the early stages of forest regeneration have residual effects on patterns of forest regeneration, but are often not available for study. The potential of pollen preserved in acidic soils to reconstruct land-use immediately before field abandonment was tested here by comparing known stand histories with analysis of pollen which had washed downward into the soil. Large amounts of Ambrosia in the soil pollen, accompanied by Asteraceae and Poaceae, indicated that a site was a plowed field prior to the start of forest regeneration. Abrupt shifts in proportions of Pinus and Quercus pollen correlated with known logging histories. Sites of unknown history included similar diversity in the proportions of nonarboreal pollen, and no evidence of logging. Proportions of tree taxa in soil pollen and vegetation surveys correlated only weakly. Soil pollen analysis, under appropriate conditions of pollen preservation, can provide evidence of major differences in site histories for which there is otherwise little evidence.

We compared success and speed of pollinium germination and putative ovule penetration in self- and cross-pollinated flowers from three clones of common milkweed (Asclepias syriaca). Event hough common milkweed is largely or entirely self-incompatible, success of selfed flowers equalled or exceeded that of crossed ones (87% vs. 74% germination, 78% vs. 63% putative penetration). This result is consistent with the postfertilization incompatibility mechanism proposed for A. syriaca. Clones different in success, and the recipient clone with the highest combined selfed and crossed success (97% vs. 74% germination for the others, 95% vs. 56% putative penetration for the others) also developed most rapidly (90% vs. 45% germination for the others 1 day after pollination; 90% vs. 13% putative penetration after 1 day). The significant among-clone variation suggests that factors intrinsic to the clones strongly affect rates of fertilization and putative penetration.

Bryophytic polsters and surface samples were collected from four sites in Shannon and Carter counties in the SE Missouri Ozarks to determine modern pollen rain. Regional pollen rain and variation in the local pollen rain are reflected by modern pollen spectra. Regional pollen rain was calculated by averaging the percentages of the various taxa in the pollen spectra from the 10 samples collected. In this area the average regional pollen rain is dominated by Pinus (18.5), Quercus (51.5), Carya (4.4) and Ambrosia (8.7). The data are consistent with the mosaic of pine-oak and oak-hickory-pine forests characteristic of this region.

I examined the relative success of pollen donors in siring viable seeds following mixed (two-donor) hand pollinations of the woody climber, Campsis radicans. Paternity was assigned using an electrophoretic marker. When pollen from two donor plants was mixed and applied to stigmas, the numbers of viable seeds sired by the two donors were generally unequal. The relative success of the two donors in a mixture was correlated with the relative success of the two donors when their pollen was applied in single-donor loads to different sitgmas on the same plant. However, performances of two donors were generally more equal when their pollen was applied in mixed loads than when applied in single-donor loads. While single-donor pollinations are often used to assess donor performance in studies of plant paternity, the above results suggest caution in extrapolating from success in single-donor pollinations to success in nature, where mixed changed, the success of the two donors changed roughly in proportion. Thus there is no basis for challenging the frequent assumption in gender allocation models that changes in pollen production produce proportional changes in male success. Relative performance of pollens in a mixture was unaffected by the total number of pollen grains deposited. Several lines of evidence suggest that postzygotic phenomena (e.g., seed abortion) are important in determining the success of different pollen donors in Campsis.