bladder fern: any fern of the genus Cystopteris characterized by a hooded indusium or bladderlike membrane covering the sori
fern: any of numerous flowerless and seedless vascular plants having true roots from a rhizome and fronds that uncurl upward; reproduce by spores
reversible process: any process in which a system can be made to pass through the same states in the reverse order when the process is reversed