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Air-dried samples of 20 hardwood species were saturated with water and subjected to thermal treatment in an autoclave at 200 °C for 2 hours under a saturated vapor pressure (1.6 MPa). Gelatinous layers were observed under optical microscope for Kalopanax septemlobus and Celtis sinensis samples, which were considered to be tension wood. The other 18 samples contained no reaction wood. Structural changes of cellulose in the cell walls by the hydrothermal treatment were studied by X-ray diffraction. The diffraction peaks of 1 1 0 and 1 1 _ 0 became closer to each other in every sample, and in many cases, merged to a single, relatively sharp peak. These phenomena suggest a change of cellulose crystalline structure from a monoclinic to a pseudo-orthorhombic system. The crystalline size calculated from the diffraction peak of 2 0 0 increased in all samples by the treatment that also induces hemicellulose leaching. Thus, the structural changes upon hydrothermal treatment are considered as a common reaction in hardwood. This phenomenon could be explained as co-crystallization of adjacent microfibrils. The d-spacing of 2 0 0 decreased by 0.8-1.6% after annealing in 17 species, while that of Kalopanax septemlobus decreased by only 0.5%, and those of Diospyros kaki and Celtis sinensis did not change by this treatment. In these 3 samples, the d-spacings of 2 0 0 were originally smaller and the crystal sizes were larger than in the other 17 samples. The proportion of peak area due to the crystalline C4 in the solid-state C CP/MAS NMR spectra suggested that the content of crystalline cellulose was higher in these 3 samples than in the other 17 samples.

Changes in Crystal Structure of Cellulose in Hardwood Cell Walls by Hydrothermal Treatment at 200 °C under 1.6 Mpa