Microscopy images. SEM and AFM images of untreated (A)-(C), hydrothermally pretreated (D)-(F), delignified hydrothermally pretreated (G)-(I) and steam-exploded wheat straw (J)-(L). In untreated wheat straw, the straw itself is surrounded by a sheath leaf (A, SEM image) and at slightly higher magnification the individual cells of the straw wall can be identified (B, SEM image). A high-resolution AFM scan (amplitude image) of a primary cell wall lining the straw cavity shows interwoven cellulose microfibrils, partially imbedded in non-cellulosic polymers (left-hand side of C). In hydrothermally pretreated wheat straw, the defibrating effect of the pretreatment causes the individual fibres to partially separate, as can be seen in D (SEM image). The pretreatment leaves a surface layer of debris and re-deposited cell-wall polymers on the individual fibres (E, SEM image). An AFM scan (amplitude image) of fibre surface shows the 'globular' deposits characteristic of lignin (F). No microfibrils are visible. Delignification of pretreated fibres causes no further separation of fibres (G and H, SEM images) but removes most of the surface layer/deposits seen in (E). Cellulose lamellae/agglomerates are now visible (H). An AFM scan (amplitude image) shows that delignification exposes intact, interwoven cellulose microfibrils (I). Steam explosion causes partially separated fibres with 90° compression bends (J, SEM image) and a surface layer with debris and droplets (K, SEM image). Droplets are indicated with arrows. High-resolution imaging of AFM shows globular surface deposits (L, amplitude image), similar to those seen on hydrothermally pretreated straw (F).