In nature tannins abound in plant tissues, especially in woody ones. Then we find them in the bark, but also in the seeds, in the leaves and in the fruits, especially if unripe.
The unpleasant taste of an unripe fruit also derives partly from its high tannin content. These molecules, in fact, are synthesized from the plant to protect it from attack by parasites, insects and herbivores. The tannins also have an antioxidant function for the plant and often protect it by isolating it from atmospheric agents.
Tannins abound in many plant drugs; among the most widely used for the amount of tannins they contain they are reminiscent of: witch hazel (leaves or bark of Hamamelis virginiana L.), bistorta (Polygonum bistorta L.), bark of oak (from various species of Quercus), walnut (leaves and mule di Junglans regia L), rhatany (roots of Krameria trianda Ruitz et Pavon) and tormentil (roots of Potentilla tormentilla)