Vegetative reproduction as related to biomechanics, morphology and anatomyof four cholla cactus species in the Sonoran Desert

Vegetative reproduction via the rooting of detached, stem segments (joints) is well recognized For certain cylindropuntias (chollas). This mode of rep roduction was characterized in the field for Opuntia acanthocarpa. O. bigel ovii, O. echinocarpa and O. ramosissima in the northwestern Sonoran Desert and the southern Mojave Desert and related to the following: (1) morphology of terminal joints and their junctions: (2) the biomechanics and anatomy o f terminal joint junctions; and (3) the rooting of detached terminal joints . Species that typically reproduce vegetatively were hypothesized to posses s mechanically weak terminal joint junctions and terminal joints with high rooting abilities. In general, resistance to failure of terminal joint junc tions depended on the diameter of the junction, with larger diameters provi ding greater resistance to mechanical Failure. Junction strength also depen ded on the presence or absence of fibres and the amount of parenchyma cells per cross-sectional area. Rooting ability appeared to depend on joint diam eter, which determines the amount of stored carbohydrates and water. Of the four species, only O, bigelovii showed evidence of vegetative reproduction in the field and was also the only species that had both relatively weak j unctions and joints with a high routing ability. Furthermore, joints of O b igelovii had the most spines per tubercle area, which increases the chance that their spines will catch on a passing vertebrate, allowing for greater joint dispersal. (C) 2001 Annals of Botany Company.