Of the four interactions, gametophytes acting on gametophytes of different species permits study of the weakest link hypothesis in an experimental design where interspecific competition is minimal. Fern gametophytes were selected as experimental organisms because they are easily cultured and amenable to the experiments' requirements of control, replication, manipulation, and minimization of competition. Six fern species usually found growing in similar natural habitats were selected for the initial survey experiment which was conducted to determine if allelopathic effects were detectable between any of the species. By similar natural habitats we mean that, among the species selected, there is some overlap between their respective ecological amplitudes so that it is possible that they would be in competition for the same space. The literature on fern allelopathy is limited but increasing. Froeschel (1953) reported that water extracts of Polypodium aureum and Lycopodium clavatum