Résumé
Tension wood xylem of arboreal eudicot species often produces unusual wood fiber, called gelatinous fiber (G-fiber). The G-fiber forms a gelatinous layer (G-layer) as the innermost layer of the multi-layered cell wall. The G-fiber generates a high tensile growth stress, which enables the hardwood species to perform the negative-gravitropic behaviors in its inclined shoot. However, the tension wood often causes various obstacles when we use the forest resources as the raw material for the timber products. Examples are the processing defects caused by its abnormal growth stress. Other examples are a high longitudinal Young's modulus and a high longitudinal drying shrinkage; their combined effect causes serious processing defects during the lumbering and drying process, e.g., distortion and cleavage of the sawn lumber, and so forth. Some researchers consider that characteristic properties of the G-fiber should be attributed to the intrinsic behaviors of the G-layer, while others emphasize the role of other lignified layer because the G-layer is often peeled off the lignified layer in the same direction during microtoming. Lately, Clair et al. revealed that detachment of the G-layer is an artifact that is caused by the stress concentration from the microtome blade, and they concluded that the G-layer is strongly attached to the lignified layer even in the oven-dried specimen. This positively supports the idea that the characteristic behaviors of the G-fiber originate from the property of the G-layer. However, the researchers still have no explanation for the generation mechanism of those G-layer properties. In our presentation, we focused our attentions on the moisture dependent changes of the longitudinal Young's modulus and the longitudinal shrinkages in two Quercus acctissima containing the tension wood xylem. And, we revealed the astonishing difference of those properties between G-layer and the lignified wall (= normal wood cell wall). Based on the obtained results, we propose a model of fine structure of the G-layer (and that of the lignified wall) to explain what is going on the drying G-layer.