Wood anatomy of Opuntias with cylindrical to globular stems

Secondary xylem of opuntias with cylindrical to globular stems is described for 30 species from the major phylads and the full range of growth habits. The majority of these cacti produce highly specialized stem and root woods that have vessel elements with alternate pits and libriform fibers. Seven mostly small caespitose species belonging to phyletic lines in North and South America have stem secondary xylem composed of vessel elements with helical to reticulate secondary thickenings and vascular tracheids with only annular secondary thickenings but no fibers. In three South American species with nonfibrous stem woods, lateral-root woods are fibrous, and Opuntia tunicata of North America exhibits fibrous xylem in outer regions of old stems. Based on small sample sizes, mean lengths and diameters of axial elements show no consistent correlations with plant size, habit, or geographical origin. Within an individual, vessel-element length and diameter are generally greatest in small lateral roots. Intrusive growth of wood fibers in trunks and main roots is proportionately greater than that in branches and small lateral roots, respectively. Interplant variations in cell types and their lateral-wall features follow gradual and predictable sequential changes, suggesting that fluctuations in levels of growth substances determine these ontogenetic patterns. Interxylary cork occurs in the outermost growth layers of very old axes in nine North American species with fibrous xylem. Woods of North American cylindropuntias and corynopuntias are more xeromorphic than previously described woods of other Cactaceae.