The Fiber
The key supporting cell in the wood of angiosperms is
the fiber cell. This is confused with the industrial term
wood fibers, which means a mash-up of all the wood cells
to make wood pulp. In plant science, the fiber is defined
as a specific wood cell type, not all wood cell types.
Fibers have few perforations, a tapering shape (like a
thin bit of plasticine you've rolled out on a tabletop), and
consist of a complex cellulose and hemi-cellulose matrix
that has been lignified, set around a central lumen. Fibers
are dead at the point of their functionality. Complex spi-
raling patterns of cellulose lie within a fiber's cell wall,
influenced by the loading that is being experienced
in that location. (A higher proportion of fibers can be
formed in highly loaded sections of a tree's anatomy.)
The path of individual fibers through the wood is
short but often it is not exactly straight: for improved
radial and tangential strength, fibers tend to develop
with a bit of interweaving. Where you will find the
straightest fiber alignments are where a long ver-
tical stem has developed in shelter, while much higher
levels of interweaving fibers are found at branch junc-
tions, bends, and in abnormal growths like burls and
galls, where fibers can be found to form arcing or cir-
cular patterns. Wood type seen here: common hazel.
The Pits
Wood has larger holes for sap flow, spaces in paren-
chyma cells for storage, and pits—many, many pits,
that allow interconnection between wood cells so that
sap and chemical signaling can be exchanged. Pictured
is the bordered pit of a larch tree—think of this as a
control point, allowing liquid exchanges between cells
but only if the torus is in the open position. The pit,
torus, and associated membrane act like a flexible
valve, unsealing only under the right conditions. This
micro-engineering at the cellular level is exquisite.
This is a mechanism by which wood cells can prevent
air from seeding through functional tissues: once air
enters a cell, the associated pits should all automatically
close due to the differences in pressure between the
aerated cell and the surrounding cells. Such micro-
controls are part of the reason that, when you dry out
wood, it can take a lot of effort to re-wet it back to its
original level of moisture content (if you ever can).
26 CityTREES
