Asa Gray, The Manual of the Botany of the Northern United States
“ Sporangia collected in dots, lines, or variously shaped clusters (sori or fruit-dots) on the back or margins of the frond or its divisions, cellular-reticulated, stalked, the stalk running into a vertical incomplete many-jointed ring, which by straightening at maturity ruptures the sporangium transversely on the inner side, discharging the spores. Fruit dots often covered (at least when young) by a membrane called the indusium (or less properly the involucre) , growing either from the back or the margin of the frond. ”
