Abstract: The result from in situ end-labelling of fragmented DNA indicated that the vessel element differentiation of the secondary xylem in Eucommia ulmoides Oliv. was a typical programmed cell death (PCD) which involved a series of events, viz. synthesis of components essential for the secondary wall formation and a well organized succession of protoplast degeneration and autolysis in the tracheary cells. The nuclei gradually became irregular with highly condensed chromatin. In some nuclei, the cistema of the nuclear envelope became unevenly dilated within which some inner membrane protrusion enclosed with nuclear materials were present. The nuclear envelope underwent disruption and the nucleus eventually degenerated. However, as the nucleus was one of the most stable components in the cell, it was among the last organelles disappeared during the autolytic process. In the process, there were two forms of degeneration in the mitochondria (Mit). In one form the Mit shrank and became disorganized; in the other, part of the matrix in the Mit became electron-lucent with breakage of the membrane nearby. The cytoplasmic component residues were phagocitized and sequestered by the dilated rough endoplasmic reticulum (RER) cisternae. The RER and vacuoles did play a vital role in the further degeneration of other organelles just similar to the lysosomes acting in the animal cells. The autolyzed debri might be utilized in situ by taking part in the formation of secondary wall or be transported to the adjacent cells through the pits.

Key words: Eucommia ulmoides, Vessel element differentiation, Programmed cell death, Ultra-structure