1. a membrane enclosing and protecting the developing spores especially that covering the sori of a fern
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.
WordNet (r) 3.0 (2006):
n 1: a membrane enclosing and protecting the developing spores
especially that covering the sori of a fern