Wisteria vines are uniquely flexible. One possible reason for this: the unusual presence of recurring rings of cambium, which are softer than the woody tissue surrounding them. This vine has been stripped of its outer cambium (bark, or bast), but an older, dark ring of cambium remains in the vine's interior, separating an inner woody core from an outer woody sheath. Steaming the vine appears to weaken the cambium ring, allowing the core to slip within the sheath, making the vine highly pliable — but less so as the vine cools and dries. Older vines have multiple successive cambium rings, creating a vine of sheaths within sheaths.