Variation in sex expression in ragweed taxa occurs to the extent that ordinarily monoecious species can exhibit dioecy. Gametophyte and sporophyte development was analyzed in two annual ragweed species, Ambrosia artemisiifolia (common) and Ambrosia trifida (giant). Seed analysis was used to obtain indirect recognition of female gametophyte and sporophyte characters. Whorls of axillary branches differentiated along the main stem axes of both species, with staminate racemes forming terminally on main and axillary branches. Pistillate heads formed in leaf axils. Pollen grains and seeds were analyzed from the base to the terminus of axillary and main stems. All characters were significantly larger in giant ragweed such as: seed mass, pollen diameter, axillary branch length, number of staminate heads per axillary branch, number of staminate flowers per head, and pollen grain number. The percent increase in giant ragweed characters compared to common ragweed ranged from 483% (seed size) to 10% (pollen diameter). Seed size revealed a greater variation than pollen size in giant, but a smaller variation in common ragweed. The size of pollen grains or seeds exhibited no recognizable trends from older to younger axillaries. The size of grains and seeds manifested no correlation at each of the different branch levels. The largest seeds differentiated at basal branches in common, and at middle branches in giant ragweed. Heaviest seeds compared to lightest exhibited 18% and 33% more mass in common and giant ragweed, respectively. However, little difference in pollen grain size was discernable in either taxon from the base to the terminus of axillary or main stems. For example, the largest grains differed in diameter from the smallest grains in common ragweed by 1% and in giant ragweed by 4%. Assemblages of pennate diatom species were associated with staminate flowers of both species.

Key words: Ambrosia artemisiifolia, Ambrosia trifida, pollen, ragweed, seeds