Observations on Functional Wood Histology of Vines and Lianas

Types of xylem histology in vines, rather than types of cambial activity and xylem conformation, form the focus of this survey. Scandent plants are high in conductive capability, but therefore have highly vulnerable hydrosystems; this survey attempts to see what kinds of adaptations exist for safety and in which taxa. A review of scandent dicotyledons reveals that a high proportion possesses vasicentric tracheids (22 families) or true tracheids (24 families); the majority of scandent families falls in these categories. Other features for which listings are given include vascular tracheids, fibriform vessel elements, helical sculpture in vessels, starch-rich parenchyma adjacent to vessels, and other parenchyma distributions. The high vulnerability of wide vessels is held to be countered by various mechanisms. True tracheids and vasicentric tracheids potentially safeguard the hydrosystem by serving when large vessels are embolized. Many vines and lianas have many narrow vessel elements (often fibriform vessel elements) in addition to wide vessel elements. The narrow vessel elements are held to increase safety. In comparing scandent with nonscandent genera within a family, one finds in some cases that tracheids occur in the scandent species, whereas fiber-tracheids or libriform fibers occur in the nonscandent ones. The same is true of vasicentric tracheids in vining species as compared to nonvining relatives; vasicentric tracheids are so abundant in some families (Passifloraceae) that libriform fibers have been lost partially or wholly. New familial records for presence of vasicentric tracheids include Cucurbitaceae and Polygonaceae, and new generic records include Antigonon, Lycopersicon, and Mandevilla. Various xylem features suggest presence of auxiliary conductive capabilities when main vessels fail. Parenchyma distributions in vines, especially those in which parenchyma surrounds fiber-sheathed vessels, may offer mechanical flexibility in vines, just as presence of wide rays does. However, starch-rich axial parenchyma adjacent to vessels may provide not merely a means of storing to facilitate growth events but, additionally, a way in which hydrolysis of starch could result in movement of sugar into vessels, permitting osmotic entry of water into given areas. Adaptations which seem to lend safety to the hydrosystem are much less in vines and lianas of perpetually moist tropical regions than they are in taxa subject to periods of drought or frost.