Exploring the evolution of humus collecting leaves in drynarioid ferns (Polypodiaceae, Polypodiidae) based on phylogenetic evidence T. Janssen 1,2 and H. Schneider 2 1 Museum National d’Histoire Naturelle, De´ partement de Syste´ matique et Evolution, USM 0602 Taxonomie et collections, Paris, France 2 Georg-August-Universita¨ t, Albrecht-von-Haller-Institut fu¨ r Pflanzenwissenschaften, Go¨ ttingen, Germany Received August 24, 2004; accepted October 15, 2004 Published online: May 12, 2005 Ó Springer-Verlag 2005 Abstract. Most species of the paleotropic fern genera Aglaomorpha and Drynaria, together con- stituting a monophyletic clade (drynarioid ferns), possess humus-collecting structures as an adapta- tion to their epiphytic life form. Humus-collectors are either present as a specialized foliar structure (external leaf dimorphism) or as a specialized leaf part (internal dimorphism). Apart from these basic patterns there are several forms of reduction and an internal fertile – sterile dimorphism in Aglaomor- pha. We present a phylogeny of drynarioid ferns based on morphological and molecular (cpDNA) markers. The genus Aglaomorpha was found to be monophyletic, whereas Drynaria is likely to be a paraphyletic assemblage including a grade of Himalayan to Southern Chinese taxa basal to Aglaomorpha. The evolution of humus-collectors is reconstructed by plotting their character state changes onto the obtained phylogeny. Despite the complex morphological pattern across species, evolution of drynarioid humus-collecting structures can be reconstructed postulating a simple sequence of character state changes based on only a few elementary processes. Keywords: Dimorphism, drynarioid ferns, epiphytism, nest leaves, phylogeny, Polypodiaceae, Aglaomorpha, Drynaria. Introduction Access to mineral nutrients limits the vigor of epiphytic plants, and several adaptive strate- gies to improve nutrient availability have been described (Benzing 1990, Zotz and Hietz 2001, Laube and Zotz 2003). One of these strategies is the housing of ants occurring in some flowering plants such as Myrmecodia Jack and some ferns such as Lecanopteris Reinw. (Benzing 1990, Gay 1993). Collecting humus is another successful strategy found in angiosperms and ferns. These plants impound litter in basket-like structures (nests) created through particular leaf arrangements. Nest- gardens harboring a diverse microfauna are known from Bromeliaceae and some ferns such as Asplenium nidus L. (Benzing 1990). In some ferns, the nest is formed by one or a few leaves that are arranged on a long creeping rhizome. The petiole of these leaves is short to absent and their lamina has a broad base that is pressed to the stem of the host-tree. This form of nests is found in Microsorum linguiforme (Mett.) Copel. and M. musifolium Blume. A further variation of the strategy is found in some ferns by differentiating some Pl. Syst. Evol. 252: 175–197 (2005) DOI 10.1007/s00606-004-0264-6