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The wall of the ripe sporangium or pollen sac is composed of a single layer of cells in most places, and these cells are provided with thickened ridges which have to do with opening the pollen sac.

This drawing together is produced by a small weal, which appears as a more or less complete pollen sac, and in the same proportion the leaf approaches the simple form of a stamen.

A true, little-modified petal is drawn together on its upper margin, and produces a pollen sac, while the rest of the petal takes the place of the stamen.