Early developmental stages of the sorus in Cyathea fulva show that the indusium is initiated from cells of the protoderm on the abaxial leaf surface, closer to the midvein than the margin of the ultimate segment. This contrasts with earlier work on Alsophila tricolor that reported a marginal origin of the indusium and possible phyletic relations with the marginal sorus in the Dicksoniaceae. This new information is considered in an assessment of the numerous forms of the indusia in five genera of the Cyatheaceae. A series of photographs shows the principal forms and the diversity of sori and indusia in Sphaeropteris, Alsophila, Nephelea, Cyathea, and Cnemidaria, including species of both the paleotropics and neotropics. The figures illustrate the inadequacy of the earlier system of classification of these plants in three main genera based upon the type of indusium. The primary role of the indusium is considered to be the retardation of water loss during sporangial development and sporogenesis.