| Abstrakt: |
In the West African Ilomba wood (Pycnanthus angolensis Exell) discolourations frequently develop after felling, which lead to serious degradation with regard to utilization. Therefore, structure and chemical composition of normal and discoloured wood were investigated in order to characterize the original wood and give a detailed account of the reactions which lead to discolourations. According to the results Ilomba, in normal condition, is all sapwood, which explains its high reactivity. The species does not have the ability to develop heartwood; only false heartwood is formed, initiated by exogenous influences. Discolourations arising in the wood after felling are reddish-brown, due to the deposition of extraneous compounds in the lumina of the ray cells. Soluble sugars disappear and the permeability decreases as a result of tyloses development. From discoloured wood only 60% of extraneous compounds could be extracted as compared to the normal sapwood, thus indicating that polymerisation of extraneous compounds takes place. Discoloured wood appears even more susceptible to decay than the bright sapwood. The pH-value increases from about 5.5 to 7.5 during discolouration, and bacteria present within the reddish-brown zones are involved in that process. |
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