Forces Necessary for Mechanical Failure of Terminal Joints of Several Species of Cylindropuntia and Opuntia (Cactaceae)

EVANS, L. S. (Laboratory of Plant Morphogenesis, Biological Sciences Research Laboratories, Manhattan College, the Bronx, NY 10471), Z. KAHN-JETTER (Department of Mechanical Engineering, Manhattan College, the Bronx, NY 10471) AND S. BUTWELL (Laboratory of Plant Morphogenesis, Biological Sciences Research Laboratories, Manhattan College, the Bronx, NY 10471). Forces necessary for mechanical failure of terminal joints of several species of Cylindropuntia and Opuntia (Cactaceae). J. Torrey Bot. Soc. 131: 311-319. 2004.The number (plant height) and arrangement of stem segments of the nearly 200 species of Cylindropuntia and Opuntia depend upon mechanical stresses at joints between stem segments in relation to the amount of internal components in joints that withstand joint failure. Stress tests were performed on terminal joints (joints between terminal and sub-terminal stem segments) of five species of Cylindropuntia and Opuntia. For these five cactus species, moment of inertia, maximum bending stress at failure, and bending moment at joint failure values ranged from 0.22 to 50.9 10-9 m4, 1060 to 12200 kPa, and 0.45 to 2.39 N-m, respectively. Results of this study show that terminal joints with larger bending stresses prior to experimentation withstood larger stresses imposed during stress testing. This result indicates that joints are 'pre-stressed' and have a finite level of resistance when they develop naturally on the plant. Results show that maximum bending stresses at failure [breaking strengthslope of M / (I/c)] were higher for tensile than for compressive portions of terminal joints and joint angle deflections changed more for compressive joint portions than for tensile joint portions during stress testing. The high resistance to bending for tensile portions corresponded to a larger total area of lignified xylem cells (the preponderant wood cells) in tensile portions of terminal joints. We conclude that the mechanical properties of tensile portions are not similar to compressive portions of terminal joints. Moreover, the breaking strength of joints was positively correlated with the amounts of lignified cells in joints. Overall, the results imply that each terminal joint for a cactus species has an innate resistance to failure (ability to withstand a specific level of mechanical stress) that is established during the natural development of the joint.