Macroscopic longitudinal shrinkage of beech and poplar tension wood is higher than in normal wood. This shrinkage is the result of mechanical interactions of cell wall layers. SEM observation of cut, dried surfaces showed that longitudinal shrinkage is much greater in the gelatinous layer than in other layers. AFM topographic images of the same cells, both in water and in air-dry conditions, confirm this result. Measurements on sections indicate around 4.7% longitudinal shrinkage for the G layer.
The gelatinous layer (G-layer) of tension wood fibres in hardwood contributes to the mechanical function of the living tree and has significant consequences on properties of solid wood. Its size, shape and structure observed by optical or electron microscopy exhibits characteristic anatomical features. However, we found that sectioning of non-embedded wood samples results in an uncontrolled swelling of the G-layer. In order to assess this artefact, the shape and thickness of the G-layer was monitored by serial sections from an embedded wood sample, from its trimmed transverse face to that located several hundreds of micrometres deep. The results revealed that the initial cutting before embedding produced a border effect responsible for the swollen nature, which is similar to sections from non-embedded material. After a conventional embedding technique was applied, a section of at least 30 micrometres below the trimming surface is required to observe an un-swollen G-layer.
To determine how gelatinous fibres and gelatinous layers contribute to the magnitude of longitudinal growth stress in tension wood, anatomical measurements of gelatinous fibres were carried out on poplar tension wood (Populus I4551). It was found that (a) no gelatinous fibres were observed under a growth strain level of 0.06 to 0.08%; (b) almost 100% of the non-conductive tissues contained gelatinous fibres above a growth strain level of 0.15 to 0.19%; and (c) the area of fibres, the area of fibres with gelatinous layers per unit of tissue area, and the thickness of the gelatinous layers predominantly influenced the magnitude of growth stress
To assess the characteristics of tension wood (TW) in Trochodendron aralioides Sieb. et Zucc., seedling stems were artificially inclined at angles of 30° (TW- 30), 50° (TW-50), and 70° (TW-70) from the vertical. At all angles, the growth promotion was pronounced on the upper side of the inclined stems, where excessive tensile growth stress was observed. A gelatinous layer (G-layer) formed in the tracheids of TW. The cell wall structure of the tracheids in TW was S1 + G. The G-layer had a small pit aperture angle <10°. TW-50 showed larger tensile growth stress, a thicker G-layer area, and a smaller pit aperture angle of the Glayer than TW-30 and TW-70. Lower levels of Klason lignin and hemicellulose and higher levels of α-cellulose content were observed in TW-50. In addition, an increase in glucose content and a decrease in xylose content in holocellulose were observed in TW-50. Therefore, it can be concluded that the degree of TW varied with different inclination angles.
Studies on the degradative ability of the blue stain fungus Lasiodiplodia theobromae (Pat.) Griffon ' Maublanc have shown several strains to cause significant weight losses (c. 20%) in wood of temperate and tropical species, aspen (Populus tremula) and rubberwood (Hevea brasiliensis), both species that commonly form tension wood. In addition to the consumption of soluble carbohydrates, major changes occurred in the ultrastructure of fibre cell walls, with a rapid attack of the G-layer of the gelatinous fibres. Following G-layer degradation, earlywood fibres of both species showed true cell wall degradation with pronounced erosion attack, suggesting that prior destruction of the G-layer afforded greater accessibility and ease of attack of the outer secondary cell wall layers.
Differentiating tension wood was observed in order to analyse the changes occurring during cell wall morphogenesis. Specimens were taken from trees in Guyana. Wall texture was analysed by means of ultrastructural cytochemistry. Modifications were encountered in fibre and vessel walls of tension wood when compared to typical wood. The changes were twofold: variation in the layering of polylamellate walls, and the deposition of a gelatinous layer in the fibre cell walls. Results are discussed in terms of variations in the rhythmic nature of cell wall deposition. Data confirm that the morphogenesis of the wall is a modular process allowing the cells to adapt to growth constraints.
Wood samples were taken from the upper and lower sides of 21 naturally tilted trees from 18 families of angiosperms in the tropical rain forest in French Guyana. The measurement of growth stresses ensured that the two samples were taken from wood tissues in a different mechanical state: highly tensile stressed wood on the upper side, called tension wood, and lower tensile stressed wood on the lower side, called opposite wood. Eight species had tension wood fibres with a distinct gelatinous layer (G-layer). The distribution of gelatinous fibres varied from species to species. One of the species, Casearia javitensis (Flacourtiaceae), showed a peculiar multilayered secondary wall in its reaction wood. Comparison between the stress level and the occurrence of the G-layer indicates that the G-layer is not a key factor in the production of high tensile stressed wood.
This paper describes the microscopic structure and morphology of stern and buttresses of swamp-grown Caryocar nuciferum L. and discusses the function of buttresses. Buttresses are mainly found at the opposite side of the leaning direction of a tree and thus could function as tension members. In contrast to the stern wood, which exhibits a moderate amount of tension wood fibres with a gelatinous layer, the wood of the buttresses on the tension side and the compression side of the leaning tree is characterised by thick-walled tension wood fibres. In addition, the number of vessels in the buttresses is substantially higher than that in the stern wood. The preferential direction of the buttresses and the anatomical differences in the various parts of the tree are discussed.
The anatomy of the secondary xylem and distribution pattern of gelatinous fibres (G-fibres) have been studied in the developing and heavy fruit bearing mature peduncles of Kigelia pinnata and Couroupita guianensis. The peduncle in both the plants developed reaction xylem as a result of growth stresses caused by development of large fruits. In Couroupita peduncles which are originally horizontal, G-fibre distribution was unilateral and similar to that of typical tension wood whereas the hanging Kigelia peduncles have uniformly distributed gelatinous fibres throughout the xylem. The tension xylem severity was higher in the basal region and decreased towards the terminal region of the current year’s peduncle but after fruit development a drastic increase in tension wood severity was observed in the terminal region. In the Kigelia peduncles, tension wood severity in terms of G-layer proportion to lignified wall was found to be less than in Couroupita. The abundance of vessels decreased with high frequency of gelatinous fibres in Couroupita. The peduncle of Kigelia is characterized by high vessel frequency, thin normal fibre walls, and thick outer walls with thin gelatinous layer in tension wood fibres. Dimensional variations were also noticed in the mechanical and conducting elements varying with tension wood severity.
Leaning trunks and branches of Trochodendron aralioides Sieb. & Zucc., a primitive vessel-less dicotyledon, show increased radial growth and gelatinous fibers on the upper side similar to the features found in dicotyledons with vessels. The patterns of peripheral longitudinal growth strain are variable among trees but similar at different heights within the same leaning trunk. Growth strains on the lower side of the trunks are very small but they are relatively large on the lower side of the branches. Growth stress in the branches is partly affected by the gravitational bending stress, which would be exerted mostly on the lower side. Large spring back strains of branches are associated with large surface strains. Both the microfibril angle (MFA) and the percentage area of gelatinous fiber show positive relationships with the measured strains. The MFA of the S2 wall layer in tracheids in the opposite wood is 24.6 ± 2.2°, whereas the MFA of gelatinous layer in the tension wood is only 14.2 ± 2.7°. The difference of MFA between the gelatinous fibers and the opposite wood is one of the factors accounting for the large contracting force for reorientation.