Scanning electron microscoPlc analysis of freeze-dried American elm wood revealed the presence of unique plugs in bordered pit chambers of vascular elements. Non-dispersive X-ray analysis of these plugs demonstrated the presence of high levels of calcium and potassium. Some pits contained only fragmentary deposits or bacteria. An hypo thesis is proposed to account for the precipitation of CaCO3 in the bordered pit chambers of vascular elements at the sapwood-wetwood boundary, resulting in the formation of amorphous plugs that fil! the pit chamber.