In order to reorientate stems and branches of (most) trees, reaction wood is
produced which provides a force in order to reorientate the tissue \citep{gardiner2014biology}.
Reorientation occurs for a number of reasons, it may be upwards as is defined by the
negative gravitropism hypothesises or for other reasons such as reorientation toward light or reducing wind drag \citep{niklas2012plant, coutts1995wind}. In softwoods this
reorientation is caused by the production of compression wood. Compression wood
forms on the lower side or bottom of the branch and expands longitudinally \citep{timell1986compression}. Hardwoods
on the other hand produce tension wood on the upper side which
contracts longitudinally \citep{gardiner2014biology}. Traditionally the
gelatinous layer (G-layer), a layer primarily consisting of low MFA cellulose
fibrils on the inside of the fibres, is credited with forming growth stresses
within tension wood. However some hardwoods produce tension wood
without producing a G-layer such as Eucalyptus nitens \citep{Qiu_2008} and a number of other species \citep{Ruelle_2006}. An intuitive way of envisioning both compression and tension wood is presented in Figure \ref{281884}.