Indusium is a membranous structure which covers a group of sporangia below it. It is differentiated, into a kidney-shaped dorsal plate called hood and a short stalk through which it remains attached with the undersurface of the pinnule at a swollen base called placenta. The sporangia attached with a placenta form group called sorus (pl. sori). Different types of sorus (simple, gradate, mixed) are seen to be present in a box like structure called sporocarp formed on the vegetative leaf, $i e$, frond of ferns.
Such spore producing structure, sorus, covered by a membranous structure is not found in any group of plants (Algae, Bryophyta, Gymnosperms) except, ferns of Pteridophyta.