Infection by Fomes annosns leads to thc formation of dry zones in conifer wood. During an investigation of the mechanism of drying it was found that water was withdrawn from infected parts of logs into uninfected parts probably by the release of hydrostatic tension. The tcnsions which exist in standing trees appearcd to be maintained in logs and to cause movement of water when the sealed tradieid system became sufficiently damaged to allow entry of air. F. annosus apparently caused this typc of clamage by enzymatic lysis in the bordered pit. However the extcnsion of dry zones beyond infection seems to imply a futher mechanism involving living xylem cells.