Macroscopic appearance and microscopic structure of bark in Indigenous South African species of Meliaceae

Abstract

Bark structure in seven South African species of Ekebergia, Nymania, Trichilia and Turraea (Meliaceae) was studied to clarify the relationships between its macroscopic appearance and anatomical traits. Like other Meliaceae, these species share the subepidermal initiation of periderm, the presence of phloem fibers, and compound sieve plates. Diagnostic bark characters were revealed to identify these taxa. All studied species share stretching bark showing conspicuous expansion without regular shedding. Their continuity is maintained by anticlinal divisions of phellogen cells and by the formation of expansion cracks, i.e., the superficial disruptions associated with new portions of the periderm. The lenticels found in all studied taxa except Trichilia, unlike expansion cracks, are derived from non-disrupted periderm. The mature bark of E. capensis shows conspicuous radial expansion of secondary phloem by periclinal divisions of axial parenchyma (proliferation tissue), which has not been reported elsewhere. Reticulate fracturing of mature bark in Ekebergia and Trichilia is presumably associated with the presence of elastic parenchymatous layers covered by a rigid periderm. We hypothesize that such elastic layers can redistribute the stressing forces of radial wood increment into the tensile forces stretching the periderm parallel to the bark surface, which cracks not only vertically but also horizontally and diagonally.