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The purpose of the present study was to investigate the ultrastructure of the outer layer of the secondary cell wall (S1) of Norway spruce (Picea abies (L.) Karst.) tracheids with emphasis on microfibril orientation. Several methods were used to obtain complementary results. Macerated tracheids and wood blocks were exposed to degradation by soft rot fungi, different chemicals and mechanical forces. Observations were made using scanning and transmission electron microscopy and light microscopy. It was concluded that the S1 layer is rather homogeneous with respect to microfibril orientation. The major part of the S1 is composed of microfibrils oriented approximately perpendicular to the tracheid axis. Evidence for a crossed fibrillar structure with microfibrils in alternate S- and Z-helices was not obtained. The transition of microfibril orientation from the S1 to the S2 layer appeared abrupt. The transitional/intermediate lamella, where a gradual change in microfibril orientation occurs, should be considered to belong to the outer part of S2.

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