Compressive/tensile stresses and lignified cells as resistance components in joints between cladodes of Opuntia laevis (Cactaceae)

The Cactaceae are a diverse group of plants with a wide variety of morpholo gies. Many species of Opuntia have segmented stems in which terminal cladod es may be separated from main-stem cladodes with varying amounts of resista nce. From a geometric approach, derivations were used to calculate normal ( axial and bending) and shear (transverse force and torque) stresses at join ts due to the weight of the cladodes. Normal and shear stresses act perpend icular and parallel to (along) the cross sections of joints, respectively. Normal stress caused by bending was >10 times that of the mean value of any other stress. Analyses were performed to determine the relationship betwee n maximum normal stress and the amount of lignified xylem cells. Such cells had thicker cell walls compared with the various other cells of stem joint s that had thin cell walls and that thus would provide the most resistance to normal stresses. An analogy was made between cactus joints and a composi te beam with reinforcing rods. In such joints, thin-walled parenchyma cells might be analogous to concrete that has little resistance to tensile stres s, while the thick-walled, lignified xylem cells would be analogous to rein forcing rods. There were statistically significant relationships between no rmal stresses (from bending and axial loads) and mean percentage of lignifi ed xylem cells (r = 0.73) and between normal stresses and total areas of li gnified xylem cells (r = 0.65) (more stress, more reinforcing xylem cells). Tensile portions of cactus joints had 23% lignified xylem cells, while com pressive portions had only 10% lignified xylem cells in joint areas (more t ension, more reinforcing xylem cells). In addition, tensile joint tissues h ad two to three times more thick-walled, lignified xylem cells in the outer 30% of the radius compared with other joint tissues types (more reinforcin g near the surface). To our knowledge, this is the first publication to pre sent mechanical stresses at stem joints of cacti and the first to relate th ese stresses to characteristics of resisting tissues in the joints of a cac tus.