Day 200: Peltigera membranacea (Membranous Dog-Lichen) provides an excellent lesson in how lichens attach themselves to a substrate by means of root-like structures called rhizines which grow out from the back side of the thallus. In Peltigera, the rhizines are large and obvious; in the finely crustose lichens which occur on rock, they may be invisible without the aid of a hand-lens and yet if you were to try to peel one away, you'd soon discover why they are also referred to as "hold-fasts." Most lichenologists agree that rhizines do not assist in tranferring minerals or nutrients to the body of the lichen, although this has not been proven conclusively. Others suggest that close attachment to the host aids in retaining moisture.
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