A thin membranous covering, especially a shield covering a sorus on a fern frond.The leaf margins themselves are often reflexed to form false indusia that protect the sori....
The latter is covered by the indusium griseum, a thin sheet of rudimentary cortex containing (on each side) the delicate medial and lateral longitudinal striae.
The upper surface of the intermediate portion of the corpus callosum is covered by a thin veil of gray substance - the indusium griseum.
Origin
Early 18th century: from Latin, literally 'tunic', from induere 'put on, don'.