We conducted anatomical studies of girdled stems of Eucommia ulmoides at various developmental stages to elucidate the origin and development of callus and the vascular cambium. In the transverse view, ray initial cells in the cambial zone began to divide both periclinally and anticlinally 2 d after girdling, Fusiform initial cells started to enlarge at 3 d, then gradually proliferated via periclinal divisions. Thus, the callus was derived from the ray initial cells of the cambial zone as well as from fusiform initial cells. In the tangential view, callus cells derived from ray initial cells were short while those from fusiform initial cells were long, thereby producing a heterogeneous structure. However, the fusiform initial cells underwent transverse divisions 10 d after girdling, which resulted in shorter cells and a homogeneous callus structure. Afterward, some short cells divided transversely while others elongated, so that a heterogeneous form was regained. Finally, the vascular meristem that was girdled early in ils development redifferentiated from short and long cells in the callus. The long cells developed into fusiform initials, with the short ones becoming ray initials.

In a comparison of cambial cells and their derivatives between naturally occurring dwarf and normal trees, vessel elements and fibers in the annual rings of dwarf trees were found to be shorter, narrower and fewer than those of normal trees. The frequency of anticlinal divisions and loss of cambial initials were low during the differentiation of xylem cells from cambial initials in dwarf trees. The length and intrusive growth of fusiform initials were slightly less than those of normal trees. Thus, it was concluded that the shortening of vessel elements and fibers in dwarf trees was due to the fact that cambial initials were themselves shortened and underwent inactive intrusive growth during differentiation of the xylem mother cells.