Reaction mechanisms for the shape control in angiosperms tension wood : diversity, efficiency, limits and alternatives
Résumé
Tension wood is known to be the main mechanism for the shape control in angiosperms. An overview of the anatomical diversity of tension wood by comparison to opposite wood is presented from the result of a study on 21 species in tropical rain forest. Anatomical measurements on vessel and fibres do not allow to evidence common criteria for the high tensile stress generation. At the ultrastructural level, microfibrils angle is the only common criterion with a lower MFA in tension wood. Typical gelatinous (G-) layer appears to be present in few species only, some other species present an intermediary stage of modification between S2 and G and in some other, fibres do not present any anatomical difference between normal and tension wood. Typical gelatinous fibres do not show any evidence of producing a higher stress level than other types of tension wood. From this study, it appears that residual tension strains reach a maximum around 3500 µstrain. The presence of a maximum is discussed; it can be interpreted either as a physical limit in the mechanism generating maturation stress or as a trade-off in order prevent the stem from internal cracks. To increase the bending moment the role of opposite wood has to be taken into account. It can be observed some very low tensile stress value and sometime compression stress value on the opposite side of tension wood. Measurement of physical properties such as longitudinal shrinkage or ultrastructural features such as MFA have been done in order to prove that these compression values in angiosperms are not artefacts. Finally, few words are presented on another complementary mechanism: the growth eccentricity with the extreme case of buttresses of tropical rain forest trees.