AbstractDry zones in conifer wood, characterised by gas filled tracheids, were induced by the injection of dilute solutions of various toxic chemicals followed by incubation at 10 or 20°C. The development of dry zones did not occur at lower temperatures nor with concentrated solutions and was prevented or retarded in an atmosphere of nitrogen. These results are discussed in relation to the development of dry zones in infections byFomes annosusand other fungi and in the formation of heartwood. It is postulated that the gradual death of ray cells alters their metabolism in such a way that gas emboli are evolved in adjacent tracheids; this can occur at sites of infection or at a distance under the influence of transportable or diffusible toxins