Résumé
The anatomy of tension wood and opposite wood is compared in 22 tropical rainforest species from 18 families from French Guyana. Wood specimens were taken from the upper and lower sides of naturally tilted trees. The measurement of residual maturation strains (growth stresses) ensures that the two samples are taken from wood tissues in a different mechanical state: high tensile growth stress wood on the upper side, considered as tension wood and low or average tensile growth stress wood on the lower side, considered as opposite wood. Thin sections, stained with the safranin / fast green method, were used for observations. We observed G layer in the tension wood of 9 among the 22 studied species. Quantitative criteria concerning fibres and vessels were measured on transverse sections of both tension and opposite wood to see if some criteria can easily discriminate these two kinds of wood. We observed a decrease in vessels frequency in the tension wood of all the species. Other criteria concerning vessels shape and area, fibres diameter or cell wall thickness do not show any general tendency between species. No systematic difference between tension and opposite wood can be observed at the fibre level. Actually more than half of them show a high tensile stress wood without G layer fibres. In this study we define "tension wood" as a high tensile stress wood, which often cannot be distinguished by traditional anatomical observation. Occurrence of G layer is always associated with mechanically high tensile stressed wood but the reciprocal is not true. Moreover microfibril angle has been estimated by X-ray diffraction. All the tension wood samples seem to have a very low microfibril angle. This shows that large differences in growth stress levels between tension and opposite wood in tropical angiosperms could be correlated with differences in wood microstructure and chemical composition.