Comparative Morphology of Reproductive Structures in Heterosporous Water Ferns and a Reevaluation of the Sporocarp

Heterosporous water ferns (Marsileaceae and Salviniaceae) are the only extant group of plants to have evolved heterospory since the Paleozoic. These ferns possess unusual reproductive structures traditionally termed ‘‘sporocarps.’’ Using an evolutionary framework, we critically examine the complex homology issues pertaining to these structures. Comparative morphological study reveals that all heterosporous ferns bear indusiate sori on a branched, nonlaminate structure that we refer to as the sorophore; this expanded definition highlights homology previously obscured by the use of different terms. By using a homology-based concept, we aim to discontinue the use of historically and functionally based morphological terminology. We recognize the sorophore envelope as a structure that surrounds the sorophore and sori. The sorophore envelope is present in Marsileaceae as a sclerenchymatous sporocarp wall and in Azolla as a parenchymatous layer, but it is absent in Salvinia. Both homology assessments and phylogenetic character-state reconstructions using the Cretaceous fossil Hydropteris are consistent with a single origin of the sorophore envelope in heterosporous ferns. Consequently, we restrict the term ‘‘sporocarp’’ to a sorophore envelope and all it contains. Traditional usage of ‘‘sporocarp’’ is misleading because it implies homology for nonhomologous structures, and structures historically called sporocarps in Salviniaceae are more appropriately referred to as sori.