Soredium(redirected from soredial)
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an organ of vegetative reproduction in many lichens. Soredia resemble tiny dust balls and consist of one or more algal cells interwoven with fungal hyphae. They are formed in the gonidial layer of the lichen, where the algal cells are concentrated. They fall through a rent in the cortical layer in the form of dust and are distributed by the wind. Upon falling on a suitable substrate, the soredia develop into a new thallus.