NOUN (2)

1. a spore-producing structure in certain lichens and fungi;

2. cluster of sporangia usually on underside of a fern frond;

The Collaborative International Dictionary of English v.0.48:

Sorus \So"rus\, n.; pl. Sori. [NL., fr. Gr. ? a heap.]
   1. (Bot.) One of the fruit dots, or small clusters of
      sporangia, on the back of the fronds of ferns.
      [1913 Webster]

   2.
      (a) In parasitic fungi, any mass of spores bursting
          through the epidermis of a host plant.
      (b) In lichens, a heap of soredia on the thallus.
          [Webster 1913 Suppl.]