| Summary: | Soral position has variously been used as a character in evaluating fern phylogeny. SEM study has shown that the leaf meristem system in tree ferns is primarily organized around a conspicuous marginal file of superficial, actively-dividing initial cells. In Cibotium the soral receptacle originates directly from the cells of the marginal initial file. The outer and inner indusia arise simultaneously early in development, on the adaxial and abaxial sides of the receptacle, respectively. In contrast, the receptacle in Dicksonia originates from an adaxially shifted segment of the marginal initial file. Furthermore, the outer indusium is initiated first, approximately at the time of receptacular differentiation, whereas the inner indusium does not become evident until after several sporangial rows have been initiated. Despite these differences, both genera demonstrate a fundamental relationship between the soral receptacle and the initial cells of the marginal meristem. Historically, a slightly abaxial position of the receptacle in Dicksonia was presupposed as a foundation of the "phylogenetic slide" in ontogeny of the sorus of tree ferns. This is shown to be in error and phylogenetic relationships based on this and other morphogenetic suppositions must be reexamined. It is proposed that a strict developmental definition of soral position may have phylogenetic significance.
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