Racemes or spikes of tiny green "bells" contain the staminate (male) flowers. Each little bell is a five-lobed corolla with five stamens.
The pistillate (female) flowers are few, and are in the axils of the upper leaves. They have no corolla, only a forked pistil with an inferior ovary, surrounded by a ribbed calyx tube which becomes the achene (seed). These flowers depend upon the wind to bring the fine yellow grains of pollen for fertilization.
Ragweed is a coarse annual with leaves deeply bipinnately dissected, lower ones opposite, upper ones alternate. The plant is named for the raggedy shape of its leaves.
It is a member of the Aster Family, Asteraceae. Strangely, its scientific name, Ambrosia, was the delicious food eaten by the mythical Greek gods to make them live forever.