Introduction

As trees grow they produce wood in order to become taller, wider or orientate stems, branches and roots. Becoming taller or reorienting stems and branches can be an effective way to out-compete other plants for light. With increasing height, width or stems becoming off axis comes increasing gravitational force, wind drag and internal pressures (for water transport), which requires either enough redundant strength in the existing structure (such as in monocotyledons) or for the tree to strengthen its structure as it increases its size. In woody plants size increase and reorientation occurs in two ways, apical and cambial growth on branches, roots and the stem(s).

Wood structure and formation

Softwoods have a simpler xylem anatomy than hardwoods, consisting mainly of axially elongated tracheids which provide both mechanical support and water transport  \citep{bowyer2007forest}. A redition of  softwood tracheid structure is presented in Figure \ref{184657}. Hardwoods are more complex with a number of different cell types. Fibres, similar to softwood tracheids provide structural support however it is their primary function, with vessels providing conduction. Vessels are comprised of multiple elements being joined at the ends to form long conduits, forming a network from the bottom to the top of the tree \citep{walker1993primary}. Rays are formed from radially orientated parenchyma cells (and rarely tracheids), see Figure \ref{639138}. Rays provide a mechanical advantage by diverting the axial force and reducing buckling and shear stresses between fibres \citep{mattheck1997wood}. More detailed wood anatomy is discussed in a number of wood anatomy texts e.g. \citet{fromm2013cellular}.