Tree stems maintain their orientation (vertical for trunks, oblique for branches) by generating asymmetrical (from one side of the stem to the other) stresses in wood, during cell wall maturation. Although the strategy used to achieve this biomechanical function is common to all the angiosperms species, i.e. generation of highly tensile stressed wood called tension wood on the upper side of the stem, the anatomical variation at the fibre cell wall level leading to the generation of large tensile growth stresses varies from one species to another.

For many studied species tension wood is characterised by the occurrence of fibres with a particular morphology and chemical composition due to the development of the so-called gelatinous layer (G-layer). This layer is essentially made up of strongly crystalline cellulose with a very low microfibril angle. However, the G-layer is not always present in tension wood. Several studies have shown that the formation of the supplementary G-layer is not constant in tension wood fibres. We observed for example tension wood that did not show anatomical difference with wood from the opposite side of the axis or peculiar secondary wall structure, with an alternance of thick and thin layers, while opposite wood has a typical secondary wall structure.

We decided to summarize various data collected trough different works considering growth strains measurement and anatomical observations in order to observe and analyse the evolution of reaction wood structure among angiosperm species.