Reaction Wood - Root and Shoot Morphogenesis
As a plagiotropic branch elongates, it might be expected to bend, downwards because of its increased weight and distance from the trunk. It is resisted by formation of reaction wood. Reaction wood is the increased xylem produced on either the upper or lower side of a branch by more rapid division of the vascular cambium on that side. In conifers compression wood reaction wood forms on the lower side and by expansion.