Wood and Stem Anatomy of Convolvulaceae

ABSTRACf Quantitative and qualitative features of wood and stem anatomy are presented for 44 collections of 16 genera and 35 species ofConvolvulaceae. Markedly furrowed xylem characterizes the genera of tribe Cresseae. Successive cambia occur in 11 of the genera studied. Large patches of axial parenchyma occur in many of these; only in one species was interxylary phloem (formed internally by the cambium) observed in the parenchyma patches. Intraxylary phloem at the periphery of the pith is universal in Convolvulaceae, but newly reported is the fact that in many species, cambial activity adds secondary phloem to the intraxylary phloem strands. These cambia were also observed to add limited amounts of secondary xylem externally in Ericybe and Operculina. Fragmentation of the vascular cylinder by growth from the inner cambia leads to initiation of radially oriented cambia (ray cambia) along the ray zone where fracture occurs. Three new types of vessel restriction patterns (nonrandom distribution of vessels in wood or absence of vessels within some parts of the secondary xylem) are reported for the family (and dicotyledons). Rays are predominantly uniseriate; ray histology and quantitative vessel features show little change ontogenetically in species with successive cambia, suggesting that this cambial mode deters the kinds of progressive changes that occur in dicotyledons with a single cambium. Vessels are much wider in lianoid Convolvulaceae than in shrubby species; the lianoid species of Ipomoea have vessels twice the diameter of those in arborescent species of Ipomoea. Conjunctive parenchyma may serve for water storage in I. arborescens and other species, but this tissue and abundance of axial parenchyma in lianas might also aid flexibility and damage resistance. Septate fiber-tracheids and septate libriform fibers occur in a few species and represent tracheid dimorphism. Occurrence of tracheids together with fibriform vessel elements in woods of many Convolvulaceae suggests relationship of Convolvulaceae to Polemoniaceae and Hydrophyllaceae; intraxylary phloem and other wood features suggest relationship between Convolvulaceae and Solanaceae.