Some Cacti Produce “Reactioncortex” Rather than Reaction Wood in Curved Sections of Weight-Stressed Shoots

Abstract: Several cacti with curved shoots had a tissue referred to here as reaction cortex. The curved shoots were at the bases of lateral branches that were initially not vertical but which became vertical as they grew, or in areas where a shoot had tilted and then its tip became reoriented upward. In these curved regions, cortex was much thicker on the convex (outer) side of the curve as compared to the concave (inner) side. Reaction cortex was located within the thick convex area; it was recognizable as having a much firmer texture than surrounding cortex, and as being more translucent or pale/whitish. Cells of reaction cortex were distinguishable as being larger, more rounded and with smoother walls than other cortex cells. Curved regions contained only a small amount of wood, but that wood was distinctly thicker on the convex side rather than on the upper concave side as would be expected of tension wood typical of eudicots. Reaction cortex was found in various species of Cleistocactus, Echinopsis, and Haageocereus among others, all members of Core Cactoideae II.