Fig. 4
FE-SEM images of fibers without pretreatment (a–c) and after pretreatment of H2SO4 (d–f), hot water (g–i), and NaOH (j–l). The photographs are obtained at magnifications of 500 (a, d, g, j; scale bar 10 μm), 5000 (b, e, h, k; scale bar 5 μm), and 50,000 (c, f, i, l; scale bar 500 nm), in which the warty layer (W) and the S3 and S2 layers of secondary wall are also shown. The same position is marked by arrowheads for each different pretreatment condition (a and b, d and e, g and h, j and k). Arrows in c, f show small globular structures and small pores present in the warty layer, respectively. Arrows in k indicates unknown substances deposited on the internal surfaces of fibers