The greater the density that spores are sown the greater the inhibition of spore germination and of protonema development in Polypodium vulgare L. The factor mainly responsible for the inhibition of germination apparently also promotes cell elongation. It does not persist in culture filtrates and is probably a volatile substance. Three other factors have been separated from culture filtrates by means of paper chromatography. All three cause a delay in germination and inhibit cell division but the evidence available suggests that at low concentrations they have a promotive effect on these processes. None of them appears to affect cell elongation.