How tree branches are attached to trunks

摘要

The vascular cambium and the growth ring it produces are continuous from trunk to branch, but the cells formed by the cambium in the upper junction of branch and trunk are oriented at approximately right angles to the normal orientation in the trunk and branch. Branch tissues begin to develop before trunk tissues early in the growing season. Maturation of branch tissues proceeds basipetally. The branch xylem is oriented downward at the branch base and encircles it to form a collar. The collar tissues meet on the trunk below the branch. The branch collar is enveloped later in the growing season by a collar of trunk xylem. Xylem in the trunk collar meet above and below the branch. Conduction into and out of the branch follows the pathway of the branch collar. The branch is structurally attached to the trunk by a series of trunk collars that envelop the branch collars every growing season. When the trunk collar was injured or removed by branch pruning, the trunk xylem above and below the cut was rapidly and extensively infected and decay developed. When pruning cuts did not injure or remove the trunk collar, no infections developed in the trunk xylem. Dye movement and the patterns of spread of bacterial and fungal pathogens also suggested that there was no local direct conduction between trunk xylem above a branch and within a branch.