TEM STUDY OF WOUND-INDUCED VESSEL OCCLUSIONS IN EUROPEAN ASH (FRAXINUS-EXCELSIOR L.)

Vessel occlusions in branches of Fraxinus excelsior L. were investigat ed by means of transmission electron microscopy. The vessel occlusions are fanned by exudates released from adjacent ray and axial parenchym a cells through the intact pit membranes. initial stages mostly displa y balloon-like structures protruding from the pit aperture into the ve ssel lumen. These inclusions possess a very electron dense outer membr ane and dispersed exudates in their interior. Therefore, the vessel oc clusions in F. excelsior do not represent true tyloses.