Abstract: Growth stress is generated in order to keep the trunk straight, or the branches back to the normal position of trees. Trees with this stress site is called reaction wood. In natural hardwood species, tension wood is formed usually when tree is tilt, or bending and partial crown. However, in the vertical trunks of plantation fast-growing species, there exists a lot of tension wood, but its structure and properties are different from normal wood which brings a series of problems in wood processing and utilization. The fibre cell wall structure of tension wood is very different from that of normal wood, especially in the inner surface of the fiber there is a cell wall layer with special morphology and chemical component, which is called gelatinous layer. This layer has many characteristics such as high content of cellulose, large crystallinity, low lignin content, small microfibril angle and abundant nanopores. Due to the occurrence of gelatinous layer is directly correlated with high tensile stress generation of tension wood, when this layers are concentrated, wood is prone to produce a series of problems such as twisting, cracking, sawing and pilling on the board surface during wood drying and processing, which seriously restricts the cultivation on high-quality plantation and the reasonable utilization of wood,resulting in a lot of economic losses. Therefore, to deeply understand the differences in tension wood and normal wood, especially the internal relations of special structure, chemical composition and biological function of gelatinous layer is the key to explore the high tensile stress generation mechanism, as well as the frontier intersection research in wood science, botany, tree physiology and materials science. This will reduce the loss caused by tension wood defects, improving the quality and utilization of wood. This paper reviews the research on the anatomical structure and structure characteristic of chemical component of gelatinous layer in tension wood, especially the research on the distribution of gelatinous fibers in tension wood, anatomical structure, pore structure and main chemical component of gelatinous layers were introduced in detail, in order to analyze the structure and characteristics of gelatinous layer during tension wood generation, as well as to provide the basis data for the mechanism of high tensile stress generation. According to the common characteristics of plantation in China such as high proportion of tension wood and uneven wood quality, the main scientific problem need to be solved in the current academia and industry is to understand the amount and ratio of species which are easy to form tension wood, the ratio of tension wood with gelatinous fibers and the mechanism of tension wood generation. At the same time, the further research can focus on the regulating mechanism of tension wood formation and development, the dynamic change of the structure and properties in gelatinous layer, the multiscale structure characterization of gelatinous layer and the study of tension wood with gelatinous layer absent. Finally, it will provide a theoretical basis for high efficient and high value utilization of the plantation forest resources in China.

Key words: tension wood, gelatinous layer, anatomical structure, pore structure, chemical component