English: A lichen - Pertusaria amara The title refers to the white lichen in the upper half of the image (the green lichen to the lower left is a different species of the same genus, Pertusaria pertusa).

Taste is not a test that would normally be applied to lichens, but the species Pertusaria amara is an exception. If a moistened finger is applied to the small white spore-producing discs on its surface (these can be seen in the photo), and then to the tongue, an extremely bitter taste will be experienced after a few seconds.

[The species name "amara" comes from a Latin word meaning "bitter" or "pungent". The equivalent Greek word is "pikros": appropriately, then, the chemical causing the bitter taste is called picrolichenic acid.]

This lichen was growing on the north-facing side of a tree.