Wound callus formation - a microscopic study on poplar (Populus tremula L. x Populus tremuloides Michx.)

Poplar trees were mechanically wounded and the subsequently built parenchymatic callus tissue and wound cambium as well as the developing lateral wound callus were investigated by light microscopy. Two strategies of callus formation wre observed. One of them is characterized by four steps: i- formation of a parenchymatic tissue at the wound edge; ii- formation of a cambium within the parenchymatic zone as a tangential extension of the undisturbed vascular cambium; iii- release of radially oriented xylem and phloem cells; iiii- formation of cambial cells with greater parallel orientation to the wound edge. The other strategy can be subdivided into three steps; i- formation of parenchymatic cells at the wound edge; ii- establishment of a wound cambium preferably within the differentiated phloem tissue; iii- formation of wound xylem and phloem cells, with the tendency to cover the wound laterally. Both strategies may occur within the same tree or even at different portions of one and the same wound.