Wood is formed by a secondary meristem called the vascular cambium as a result of a complex interaction between environment and programmed cell development that is sensitive to both genotype and microenvironment. Wood formation involves both cell division and cell differentiation including cell expansion, secondary wall formation, lignification, and programmed cell death. As a tree gets older, the type of wood that is formed changes. During the first 10 or so years of growth, juvenile wood is formed, which is characterized typically by fast growth rate, low basic density, low stiffness, short fibers, and more abundant reaction wood. Wood has two main physiological functions: to conduct water from roots to leaves and to support the branches and crown in large trees. The wood formed by angiosperm trees shows a much greater diversity of structure and hence a more complex process of wood formation than the wood formed by conifers.
Donaldson, L., Nanayakkara, B., & Harrington, J. (2016). Wood Growth and Development. In Encyclopedia of Applied Plant Sciences (Vol. 1, pp. 203–210). Elsevier Inc. https://doi.org/10.1016/B978-0-12-394807-6.00114-3