AbstractThe single germ tubes produced by the spores ofEphemerumcomprise highly chlorophyllose cells divided by transverse cross-walls. All subsequent cross-walls are oblique and thus the protonemata ofEphemerumwould appear to be almost entirely caulonemal. Initially, the protonema comprises a loose mat of irregularly-branched filaments. Subsequently, leading filaments around the margins produce upright determinate branching systems which give the protonema ofEphemerumits distinctive appearance. The ultimate ramifications of these systems terminate in sharp points with thickened walls whilst their regular branching is predetermined by zig-zag development of the young side branches. Unlike all the other parts of the protonema, the determinate branching system fails to regenerate when transplanted onto new medium. Whereas cytokinins stimulate bud formation and cause the formation of highly attenuate filaments, abscisic acid induces the production of chains of mucilage-invested spherical brood cells, a phenomenon also seen in ageing cultures. These are desiccation resistant and germinate rapidly on new medium. Nutrient-free agar has little effect on protonemal morphogenesis.