The ac ve genera on of stress in maturing wood plays an essen al role for the biomechanics of trees. Their main func on is to act as a motor system, enabling the postural control of the tree and adap ve reorienta ons. At the macroscopic level, the mechanism underlying this func on is well-known: the asymmetry of stress magnitude around the tree periphery generates a bending moment that may counteract the bending moment induced by the increasing self load or induce changes in stem curvature. In angiosperm trees, the asymmetry of matura on stress is generally achieved thought the differen a on of tension wood (TW) on one side of the tree. However, the underlying mechanism at the microscopic level (why and how TW develops a tensile stress during its matura on) is not yet fully understood. Actually, the rela on between the structure and func on of tension wood is puzzling: its func on consists in shrinking longitudinally, but one its characteris c structural feature is to have a gela nous layer (G-layer) cons tuted of a large amount of s ff crystalline cellulose oriented almost parallel to the cell axis. This design seems consistent with its func on only if tensile stress results from the shrinkage of cellulose. The paradox lies in the fact that cellulose in wood has a defined crystalline configura on and a great chemical iner a, so that it is difficult to imagine a mechanism by which it would spontaneously shrink a*er its incorpora on into the cell-wall. A revival of interest on this issue during the last ten years yielded a number of experimental results on the fine structure, biochemistry, topochemisty, mechanical behavior and in vivo mechanical state of TW, as well as a number of hypothe c mechanisms that would generate the tension. Our aim is to iden fy which mechanism best describes the real process. This presenta on will briefly review exis ng results and hypotheses, and use micro-mechanical models to discriminate between them. Evidences that the in vivo tensile stress is supported by the microfibrils of the G-layer will be presented, and mechanisms consistent with this fact will be analyzed with respect to their consistency with other observa ons and mechanical considera ons.
