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SEM images of cross section birch outer bark showing the effect of each type of pretreatment on the structural configuration of cell layers. (a) untreated material; (b) hydrothermal pretreatment (HT); (c) steam explosion (SE); (d) organosolv pretreatment without catalyst and (e) organosolv pretreatment with alkaline catalyst. Magnification 2500×.
