Lichen asexual-reproduction structure.
Soredia algal cells that have been encased within hyphal tissue. Thus protected, and otherwise present on the lichen surface, they are able to break off from the parental lichen to found new lichen colonies elsewhere.
Because lichen exist as two separate species, the growth of those two species must be coordinated both within the lichen and in terms of the reproduction of the lichen, that is, the production of the equivalent of progeny lichen.
It is the equivalent to the offspring taking off for new "" with a sampling of their parent's seeds and animals such that a new based on the old farm's may be established elsewhere. The soredia are those offspring in combination with those "seeds".