Breakdown of End Walls in Differentiating Vessels of Secondary Xylem in Quercus rubra L.

Formation and breakdown of end walls were studied in the vesselsof the secondary xylem of Quercus rubra L. The end walls inradially expanding vessel members were very thin but showeda three-layered structure— two peripheral layers and adarker central layer. When longitudinal walls of vessel membersformed secon dary walls, the end walls had thickened considerablyand acquired secondary walls on their periphery. The disintegrationof end walls occurred at about the same time as the disintegrationof the vessel proto plasm. Frequent observations of intermediatestages in the disintegrating end walls indicate that breakdownis a gradual process brought about by the activity of vesselmembers' protoplasm.