MIKESELL, JAN E. Department of Biology, Gettysburg College, Gettysburg, PA 17325. - Comparison of gametophyte and sporophyte development in ragweed species.
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