Primarily this work focuses on fibres as they are the structural cells expected
to be responsible for growth stresses in normal and reaction wood within
hardwoods \citep{archer1987growth}. Fibres consist of a number of cell wall layers. Normal wood fibres
consist of a middle laminar (ML, connecting the fibre to
the surrounding cells) and a primary cell wall (P) during expatiation and a secondary cell wall (S) consisting of
S1, S2 and S3 sub-layers formed once expatiation is complete (produced in chronological order so the overall composition
will change depending on the cells developmental stage) \citep{barnett1981xylem}. The S2 layer is
the thickest layer and consists of cellulose macrofibrils wrapped in a steep helix
around the cells longitudinal axis. The cellulose is contained within a
matrix of hemicelluloses and lignins giving the cell wall properties
of a reinforced composite \citep{niklas2012plant}.
In order for the living cambial cells to produce wood, each cell must go through
division from its parent cell, growth and death. Because the cambium (and apical
meristem) are continually dividing it allows for the tree to be a dynamic
structure changing its form to become better adapted to its changing
environmental setting even though large portions (ie the wood) are dead.
Cell division, formation, elongation and death
Dicotyledons trees and gymnosperms grow in two main ways, upward by apical growth and
outward by cambial growth. As the cambium is forming, fusiform and ray initials
are created from the apical meristems. From the cambial initials,
cells to the inside create the elements of xylem (tracheids, vessels,
fibers, parenchyma, etc.), while cells to the outside become phloem \citep{fromm2013cellular}.
During primary wall formation rapid elongation occurs. The internal hydrostatic (turgor) pressure causes cell expansion controlled (among other reasons) by the orientation of the cellulose microfibrils \citep{Tyerman_2002,16261190}.
Because the centre of the cell has restricted movement, elongation
(to dissipate the increasing tensile forces from the turgor pressure)
occurs by
tip growth \citep{taiz2006plant}.
The primary cell wall (P) has randomly orientated MFs embedded
in hemicellulose and pectic compounds and becomes lignified when the secondary cell wall (S) layers are added.
The ML is highly lignified. Often the term compound middle laminar (CML) is used to describe
the ML and P at once as it can be hard to distinguish between them. Once the cell has reached its
full size biosynthesis of the S layers starts \citep{fromm2013cellular}.
Typically the S1 layer is thin and comprised of microfibrils winding around the
cell axis at a high angle. Within the layer many laminates are found.
Within each laminate the MFs are closely aligned, however between each laminate
they can (but do not necessarily) differ, or even reverse the direction
of the helix, although lower right to upper left
orientation tends to be favored \citep{fromm2013cellular}. Close to the S2 layer the MFA decreases
rapidly. The S2 layer bound to the inside of the S1 is typically much thicker and
has more vertically oriented MFs compared to the primary, S1 and S3
layers, these MFs circle the cell axis from lower left to upper right. The thin S3 layer formed on the inside of the S2 layer is produced with
high MFA, reversing the direction of the MF helices to lower right to upper
left \citep{walker1993primary}.
In tension wood of some species a gelatinous layer (G-layer)
is formed on the inside of the innermost wall (S1, S2 or S3) \citep{gardiner2014biology}. The
G-layer has near axially oriented microfibrils and very little lignification.
It is suspected that the G-layer plays an important role in the generation of
reorientation stresses \citep{Pilate_2004}.
At some point during the formation of the secondary cell wall, or soon after, the
cell shrinks vertically and expands tangentially \citep{Boyd_1972} (with the exception of
some young softwoods, where the opposite occurs \citep{jacobs1945l}). Because of the connectedness
between cells a stress profile forms within the stem. After the secondary wall formation cell death
occurs.
Cells and wood in the context of a whole tree
Growth stresses in trees develop as part of cell formation and are thought to provide a
superior mechanical structure \citep{mattheck1997wood}. The continual formation of new cells contracting
on the periphery of the stem causes the older wood which has completed formation
to contract with each new layer of cells. Older wood near the centre
of the stem becomes compressed while the newer cells cannot fully contract and
remain in tension \citep{Archer_1987}.
Growth stresses in normal wood increase the mechanical stability of the stem by increasing resistance to compression failure, which occours at a smaller deformation than tensile failure. In reaction wood growth stress provides the ability for the stem to reorient in
order to be best adapted to its environment.Growth stresses allow for an adaptive organism to survive in a changing environment,
however they also cause significant value loss when harvesting and
milling timber \citep{kubler_1987}.