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}.