Wood of a member of the grape family (Vitaceae), Cayratia, shows wide, flexible ray areas separating the fibrous plates. In the fibrous plates are a few wide vessels and many narrow vessels: vessel dimorphism
LIANAS AND VINES [ PDF ]
Lianas (woody vines) and vines have several requirements where wood structure is concerned, requirements that in part run counter to each other. Lianas and vines have the advantage of getting into the light without forming their own stem support system—they use plants they twine or lean on as their support system. They can produce less wood and fewer branches in the process of growing rapidly upward. With less wood, they compensate by having wider vessels. But wider vessels are vulnerable—any one vessel, if disabled by air embolisms would represent a loss in the conductive system. How to safeguard the wide vessels? An unusually high proportion of lianas have true tracheids (= tracheids as the ground tissue of wood) or vasicentric tracheids. Are these mechanisms for conductive safety as important in lianas (and less woody vines) as in chaparral and desert shrubs? Yes, although that hadn’t been appreciated. Lianas had not been much studied with respect to wood anatomy because—once again—people don’t build houses or furniture from wood of climbing plants. Vessels in lianas are rather large and potentially vulnerable, so having a subsidiary conductive tissue sheathing the vessels makes sense. The conductive efficiency of wide vessels in lianas had been appreciated, but the idea that an appreciable number of lianas have conductive safety in the form of tracheids and vasicentric tracheids was new.
Not all vines have tracheids or vasicentric tracheids, but conductive safety can be achieved in other ways. Vessel grouping, for example. How to have the advantages of vessel grouping when there is only space for a limited number of wide vessels? For a number of vines and lianas, the answer is a sort of vessel dimorphism. Having narrow vessels grouped with the wide vessels. Narrower vessels are less likely to embolize than wide ones, but by being grouped with the wide ones, narrower vessels can maintain the same conductive pathways when the wide ones are disabled. Theoretically, this system works best if there are few wide vessels, many narrow vessels. If the widths are measured and graphed, there may not be two peaks—dimorphism doesn’t mean that. Division of labor doesn’t mean bimodality. Bimodality is a mathematical construct, but division of labor is a process done by plants independent of how people construct graphs.
Another mechanism of achieving conductive safety is sheathing of vessels with fibrous tissues, between which are soft tissues. Cable construction, this could be called. Sometimes vines and lianas have wide, tall rays with thinner walls, separating the vessel-bearing fascicular areas (Aristolochia, Clematis). Sometimes vines and lianas have fibers sheathing vessels individually, with axial parenchyma around the fiber strands (Thunbergia). And successive cambia provide ways of interspersing soft tissue and hard tissue, with vessels in the hard tissue (Antigonon, Anredera, Bougainvillea). Thin walled rays that offer flexible tissue between plates of vessel-containing fibers provide a form of cable construction. Cable construction types permit twisting and shifting of the vessel-bearing parts of the wood as the vine changes positions in its environment, all the while protecting the vessels.
Vines and lianas, like herbaceous plants, are not the stuff from which houses are made. Consequently, understanding of their wood anatomy has lagged behind that of forest trees. Just as I sensed shrubs and herbaceous plants needed more study early in my career, I sensed that vines and lianas needed more study, and I included them in a number of my papers. In a sense, vines and lianas teach us about nonvining plants by the wood differences vines show in comparison with conditions in self-supporting woody plants.
In studying wood of vines and lianas, we find some themes repeated independently in many groups (wide vessels, wide flexible rays), and in a smaller number of groups (successive cambia; loss of bars on perforations plates). In plant structure, there usually are several ways of meeting the structural and conductive requirements of a plant. Some themes, like wide vessels, recur in most species of climbing plants. But other modes relate to individual characteristics of particular vines or lianas and what they do in nature. Finding both common features and features that differ is important, and relating them to the habit and habitats is just as important in climbing plants as in nonclimbing plants.
Because vines and lianas are not useful as timbers, their woods are collected less often than those of trees, and xylaria have proportionately few specimens of them than tree woods. Also, alarmingly, liana woods in xylaria are more often incorrectly identified than those of trees. Associating the flowering materials, collected from the canopy, with the right stem seen at ground level, is not always easy. Funding for research has not favored study of wood of lianas and vines, just as funding has not favored study of other types of non-tree species. More importantly, our view of what woods is like tends to be skewed in terms of tree species. When one reads the older generalizations about woods, one is really reading generalizations about tree species more often than one might think.