Primary thallus
primary thallus (pl. primary thalli) – the first fully differentiated thallus that is formed. Most lichens only form a primary thallus, and no secondary thallus. Some genera, especially Cladonia, have a crustose or squamulose primary thallus and a fruticose secondary thallus. Some predominantly crustose or foliose lichens may secondarily also develop fruticose thallus parts. For example, Caloplaca thamnodes forms a placodioid thallus in sheltered microhabitats but develops a coralloid thallus if exposed to sea spray. Most lichens, however, only form a primary thallus, and a secondary thallus is absent.
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