The Collaborative International Dictionary of English v.0.48:

Indusium \In*du"si*um\, n.; pl. Indusia (-[.a]). [L., an under
   garment, fr. induere to put on: cf. F. indusie the covering
   of the seed spots of ferns.] (Bot.)
   (a) A collection of hairs united so as to form a sort of cup,
       and inclosing the stigma of a flower.
   (b) The immediate covering of the fruit dots or sori in many
       ferns, usually a very thin scale attached by the middle
       or side to a veinlet.
   (c) A peculiar covering found in certain fungi.
       [1913 Webster]

WordNet (r) 3.0 (2006):

indusium
    n 1: a membrane enclosing and protecting the developing spores
         especially that covering the sori of a fern