A type of sorus (or group of sporangia) in ferns (e.g. in Hymenophyllaceae), in which there is a sequence of development of ripe sporangia from apex to base of the sorus. This contrasts with the simple sorus (e.g. in Osmundaceae) in which there is simultaneous ripening of all the sporangia, which is regarded as primitive; or with the mixed sorus (e.g. in Polypodiaceae) in which there is irregular ripening of later sporangia among earlier ones, which is regarded as advanced.
Subjects: Plant Sciences and Forestry.