Honey mesquite (Prosopis glandulosa [Torr.] var. glandulosa) stems were examined to determine the location of gum deposits within the wood and bark. Trees on field sites had been treated either with ethephon (2-chloroethylphosphonic acid) or 2,4,5-T ([2,4,5-trichlorophenoxy]acetic acid) or remained untreated. Gum deposits were of two types. The most obvious were gum cavities in the wood which often extended radially through the bark to the stem surface. These were caused by local destruction of a portion of the vascular cambium and were associated with tissue disruption and cell proliferation. Vessels and xylem parenchyma cells associated with cavities were filled with gum. Similar results were experimentally obtained in greenhouse plants following injury to the vascular cambium. A second type of gum deposit was found in the phloem. These gum pockets existed as discrete, tangentially elongated, gum-filled zones in the axial system of cells. Gum pockets were formed by the breakdown of axial phloem elements...