Ultrasound acoustic emissions from bark samples differing in anatomical characteristics

This study examines ultrasound acoustic emissions (UAE) from various bark types differing in anatomical characteristics. UAE were detected in dehydrating bark strips from twigs of two conifers (Pinus nigra ARNOLD, Taxus baccata L.), and four woody dicotyledons (Hedera helix L.. Malus sylvestris MILL., Sambuctis nigra L., and Tilia platyphyllos SCOP.) with varying contents of dead mechanical elements in the cortex and in the secondary phloem. If filled with water, non-conducting bark elements (sclereids, fibre-sclereids, secondary phloem fibres and periderm cells) emitted ultrasound during dehydration. Signal production varied between the species and the bark layers studied. Pressure infiltration increased the number of UAE conspicuously. UAE were also registered from infiltrated sections of bottle cork (Quercus suber L.) and infiltrated walnut shells (Juglans regia L.) built of sclereids only and characterised by lignified secondary cell walls. Stems of Sphagnum sp. emitted UAE, probably originating in hyalocysts, dead non-conducting water storage cells in the leaves. Living cells (inner epidermes of bulb scales of Allium cepa L. and collenchyma, strands from the herbaceous stern of Lamium maculatum L.) produced only very few UAE. The results suggest that ultrasound acoustic emissions during dehydration do not only occur in cavitating conducting xylem elements but also in non-conducting, dead cells with thick walls.