Source : Webster's Revised Unabridged Dictionary (1913)

Sorus \So"rus\, n.
      (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.

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

Source : WordNet®

sorus
     n 1: a spore-producing structure in certain lichens and fungi
     2: cluster of sporangia usually on underside of a fern frond
     [also: {sori} (pl)]