At the left you see a drawing of a ragweed pollen grain. These grains are small -- 40 grains laid end to end would span about one millimeter, or 1/25ths of an inch. A single ragweed plant can produce a billion airborne pollen grains during an average season You can imagine the effects of this if you know that as few as 20 ragweed pollen grains per cubic meter of air can trigger a sneezing fit! Ragweeds begin producing pollen when the days start getting shorter in late summer. Unless the plants are removed they keep producing pollen until it frosts.
Of course pollen grains carry the male germ cell, and are released from the anthers of stamens, which are a flower's male sexual parts. As the illustration at the top, right shows, on ragweed plants male flowers are in one place while female flowers are at another. Male flowers are arranged in pagoda-like racemes at the top of the plant. The picture at the left shows a close-up of a raceme section. At the right you see an even closer shot showing the individual flower heads. The granular, gray items beneath the green involucres are pollen-producing stamens.
As the top, right picture shows, female flowers occur below the male ones, mainly in the axils of leaves. Several female flowers are shown at the left. The slender, string-like items are styles arising from ovaries, reaching out into empty space to catch pollen grains. As with other flowering plants, the ovary will develop into a fruit, which in the case of ragweeds is a nutlike or burlike thing bearing low, blunt "tubercles" or spines. Usually they're about the size of a BB or a pea.
Ragweed fruits are important food for many animals. They are favorite foods of Goldfinches, Juncos, Redwing Blackbirds, Bobwhites, Redpolls, White-crowned and White-throated Sparrows, and also are often eaten by certain mammals, such as Thirteen-lined Ground Squirrels and Least Chipmunks.