Relationships between Number of Stem Segments on Longest Stems, Retention of Terminal Stem Segments and Establishment of Detached Terminal Stem Segments for 25 Species of Cylindropuntia and Opuntia (Cactaceae)

Abstract

of detached terminal segments that become established for 25 cactus species. We hypothesized that cactus species with more segments per stem should require more force to remove terminal segments. Regression analyses of mean maximum stress at failure of terminal joints versus mean number of segments for the species tested gave a line equation of y = 380x + 1350 with a regression coefficient of 0.85. We hypothesized that cactus species with few segments per stem for adult plants should have good establishment of detached terminal stem segments while species with many segments on adult plants should have low levels of establishment of detached terminal segments. Regression analyses of mean establishment of terminal segments as a function of mean number of segments gave a line equation of y = - 1.73x + 80.6 with a regression coefficient of 0.77. In contrast, plant height was not related to either maximum normal stress at failure or segment establishment. We conclude that number of stem segments on longest stems of plants was a good predictor of force necessary to remove terminal segments. Number of stem segments was also a good predictor of rooting and establishment of terminal segments. Overall, the data of this study show that Cylindropuntia and Opuntia species with large numbers of stem segments along each stem rely mostly on sexual reproduction for procreation while species with fewer stem segments rely mostly on asexual reproduction via rooting of detached stem segments.