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• weed (Fig. 179), smartweed, buckwheat, elm are

examples.

Fig. 179.—Knotweed, a very common but inconspicuous plant along hard walks and roads. Two flowers, enlarged, are shown at the right. These flowers are very small and borne in the axils of the leaves.

Some flowers lack the pistils: these are staminate, whether the envelopes are missing or not. Others lack the stamens: these are pistillate. Others have neither stamens nor pistils: these are sterile (snowball and hydrangea). Those that have both stamens and pistils are perfect, whether or not the envelopes are missing. Those that lack either stamens or pistils are imperfect or diclinous. Staminate and pistillate flowers are imperfect or diclinous.

Fig. 180.—Staminate Catkins Of Oak. The pistillate flowers are in the leaf axils, and not shown in this picture.

Fig. 181.—Begonia Flowers.

Staminate at A: pistillate below, with the winged ovary at B.

When staminate and pistillate flowers are borne on the same plant, e.g. oak (Fig. 180), corn, beech, chestnut, hazel, walnut, hickory, pine, begonia (Fig. 181), watermelon,