It is shown that 3-indolylacetonitrile, at 10 mg./l., induces great internode extension in gporelings of Marsilea , in aseptic culture, even in media (4 per cent, glucose or 2 per cent, glucose+2 per cent, mannitol) in which without IAN the internodes are almost completely suppressed. The nitrile also retards the hetero-blastic leaf development and inhibits the development of morphological features characteristic of land forms. The effects of the nitrile on extension growth are discussed in relation to the relevant literature. The other effects of the nitrile are also discussed and it is concluded that they are secondary consequences of the effect of the nitrile on extension growth.