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.]

WordNet (r) 3.0 (2006):

sorus
    n 1: a spore-producing structure in certain lichens and fungi
    2: cluster of sporangia usually on underside of a fern frond