WHY VINES HAVE NARROW STEMS - HISTOLOGICAL TRENDS IN BAUHINIA (FABACEAE)

Xylem (wood) tissue in plants functions both for mechanical support and water transport. Since vines are mechanical parasites, they allocate less biomass for xylem tissue than do free-standing trees or shrubs. Within the genus Bauhinia, stems of vine species were found to have not only less xylem per distal leaf area, but also less phloem and cortical tissue when compared to tree and shrub species. The phloem and cortical reductions are interpreted as an indirect effect of the developmental/geometric constraints imposed by the evolution of a reduced mechanical system. Apparently vines overcame these constraints with the evolution of wider vessels and wider sieve tubes and with many types of variant (anomalous) secondary growth. The long and wide vessels of vines, which compensate hydraulically for the reduced xylem areas, may help limit the distribution of vine species in nature.