Some lichens contain both green algae and cyanobacteria. In these cases, the cyanobacteria may be enclosed in cephalodia. These outgrowths of the main thallus can take many forms. The cyanobacteria which live in cephalodia can function in nitrogen fixation on behalf of the lichen. These cyanobacteria are generally more rich in nitrogen-fixing cells called heterocysts than those cyanobacteria which may live in the main thalli of lichens.
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