Axillary Wood
In all these images, I have been showing frac-
ture surfaces of broken branch junctions, so
some of the images in this article have included
glimpses of some axillary wood, which I have cate-
gorized as a reaction wood (Slater et al., 2014).
As with all reaction woods, there is a drift in tissue
qualities as there is a graduated transition from
'normal' wood to a reaction wood. Axillary wood
is most distinctly different in its center, right in the
center of the branch union: cutting further and further
into a branch junction is a bit like cutting into the
middle of an onion. The tissues are changing qualita-
tively as you get nearer and nearer to the center.
Pictured is some axillary wood of common beech
when I fractured a branch junction. Probably the
most important practical aspect of the twisting and
turning that the wood grain of axillary wood does is
that it causes some wood cells to be orientated so
that you must stretch them and break them along
their length to break the branch junction. This requires
a great deal more energy to break this wood than if
all cells were orientated radially or tangentially.
Axillary wood is the latest key finding in wood
anatomy, from an arboricultural perspective. It is a
key responsive woody tissue that is much denser than
surrounding wood, much more crack-resistant, much
stronger in the radial and tangential directions—but
not much use in the conduction of sap (Slater et al.,
2014). As a tissue type formed in all trees which have
branches, it warrants much more scientific scrutiny.
It has been my pleasure to introduce its importance
into arboricultural texts which have, for many years,
ignored its existence or not realized its significance.
That is why they awarded me a PhD—that I had iden-
tified an important biomechanical role played by this
key woody tissue, which helps to form the branching
structure of trees. It is easy to find if you know what
you are looking for, as it is the denser wood that is
typically much more difficult to split with an axe,
that lies in the crotch of a branch junction due to the
swirling character of the wood grain in that location.
The consequences of understanding the role of axil-
lary wood in trees are many, so I invite readers that
found this short journey to 'Planet Wood' inter-
esting to follow up with some of my publications
that help to define the properties of axillary wood. I
wonder how many more lessons can be learnt from
close examination of the wood that trees grow.
30 CityTREES
