Format: ORAL

Alexei Oskolski

Department of Botany and Plant Biotechnology, University of Johannesburg, Johannesburg, South Africa

The relations between macro- and microstructure of bark are poorly explored; the most important contribution has been made by T.C. Whitmore (1962). The bark appearance is largely determined: (1) by the ability of the outermost layers of bark to maintain its continuity in the course of dilatation, and (2) by the presence or absence of separation layers, i.e. the layers of fragile tissues enabling a regular abscission of outer portions of bark.  Diverse barks can be classified into four major architectural types (stretched, exfoliating, furrowed and peeling barks) by possible combinations of these conditions. Stretched (e.g., Adansonia digitata) and exfoliating barks (e.g.  Celtis africana) share the ability to conspicuous expansion, usually with lenticels and eye-like markings. The stretched barks have smooth surface, whereas the exfoliating ones bear superficial scars of fallen scales. These barks share a thin periderm underlaid with parenchymatous layer.  Supposedly, these architectural types are associated with photosynthetic ability and gas exchange between stem tissues and environments. The furrowed (e.g., Quercus suber) and peeling (e.g., Buddleja saligna) barks share prominent vertical fissures, without lenticels and eye-like markings.  The furrowed barks lack separation layers; they are mostly covered by conspicuous persistent rhytidome or thick single periderm. Such bark architecture can be associated with fire protection as well as with decline of litter accumulation.   The peeling barks are distinctive by detachment of relatively thin strips. Their periderms usually serve as separation layers whereas the collapsed phloem perform protective function; occasionally (e.g., Vitis vinifera), the functions of periderm and phloem are allocated in the opposite way. The peeling barks are hardly suitable for colonization by epiphytes.   Several ontogenetic trends in age transformation of bark types were found. Estimation of bark architectural types along with the bark thickness is a promising approach in ecological research.