Page 370 - Biosystems Engineering

CHAPTER 12

Evaluation of

Transgenic Wood for

Wood Productivity

and Quality

Siqun Wang
Department of Forestry, Wildlife & Fisheries
University of Tennessee
Knoxville, Tennessee

12.1 Introduction
Wood is a cellular solid characterized by a high degree of anisotropy
at all structure levels. It consists of different cell types that are oriented
in the axial direction (tracheids in softwood and libriform, tracheid
fibers, and vessels in hardwood) or in radial direction (ray cells). The
cell walls of wood fibers are built up of layers of different thickness.
Each layer has different spiral microfibril angles (MFA) and chemical
components. The secondary cell wall layer (S ) is the predominant fac-
2
tor that determines the mechanical properties of the fiber because its
thickness accounts for approximately 80 percent of the thickness of the
entire cell wall. The stiffness of wood mostly relies on the semicrystal-
line cellulose microfibril as found in Z-helix form around the lumen
within the cell walls of the wood (Meylan and Butterfield 1978) as well
as its microfibril angle (Cave 1968; Tze et al. 2007). As a natural mate-
rial, wood has been used for homes and other structures, furniture,
tools, vehicles, papers, decorative objects, and biopolymers.
There is no doubt that America is experiencing its second major
energy crisis in 30 years, and bioenergy from renewable resources
will be a very promising solution. Ethanol as an alternative fuel is

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