Biomechanics of the columnar cactus Pachycereus pringlei

We report the longitudinal variations in stiffness and bulk density of tiss ue samples drawn from along the length of two Pachycereus pringlei plants m easuring 3.69 and 5.9 m in height to determine how different tissues contri bute to the mechanical stability of these massive vertical organs. Each of the two stems was cut into segments of uniform length and subsequently diss ected to obtain and mechanically test portions of xylem strands, stem ribs, and a limited number of pith and cortex samples. In each case, morphometri c measurements were taken to determine the geometric contribution each tiss ue likely made to the ability of whole stems to resist banding forces. The stiffness of each xylem strand increased basipetally toward the base of eac h plant where stiffness sharply decreased, reaching a magnitude comparable to that of strands 1 m beneath the stem apex. The xylem was anisotropic in behavior, i.e., its stiffness measured in the radial and in the tangential directions differed significantly. Despite the abrupt decrease in xylem str and stiffness at the stern base, the contribution made by this tissue to re sist bending forces increased exponentially from the tip to the base of eac h plant due to the accumulation of wood. A basipetal increase in the stiffn ess of the pith (and, to limited extent, that of the cortex) was also obser ved. In contrast, the stiffness of stem rib tissues varied little as a func tion of stem length. These tissues were stiffer than the xylem in the corre sponding portions of the stem along the upper two-fifths of the length of e ither plant. Tissue stiffness and bulk density were not significantly corre lated within or across tissue types. However, a weak inverse relationship w as observed for these properties in the case of the xylem and stem rib tiss ues. We present a simple formula that predicts when stem ribs rather than t he xylem strands serve as the principal stiffening agents in stems. This fo rmula successfully predicted the observed aspect ratio of the stem ribs (th e average quotient of the radial and tangential dimensions of rib transecti ons), and thus provided circumstantial evidence that the ribs are important for mechanical stability for the distal and younger regions of the stems e xamined.