Chapter 5 The woody cell wall
木材细胞壁
Ⅰ. Chemical structure
化学结构
Ⅱ. Layering structure
壁层结构
Ⅲ. Cell wall sculpturing
细胞壁上的特征
Ⅰ. Chemical structure
1. Thickening of the cell wall（细胞壁增厚）
Bundles of cellulose chains are continuously laid to the inside
of cell walls, and this makes the cell wall thicker and thicker.
2. Microfibril structure （微纤丝结构）
Non-crystalline area
Crystalline area
3. Functions of cellulose, hemicelluloses and lignin
A piece of wood can be analogized as a block of reinforced
concrete. In this analogy,
Cellulose: the framework – the thick steel rods
Hemicelluloses: the connecting agents – the thin iron wires
Lignin: Encrusting substance – the paste of cement and sands
Ⅱ. Layering structure
W: Warty layer
S3: S3 layer
S2: S2 layer
S1: S1 layer
Pr: Primary wall
ML: Middle lamella
cell wall layers
main components
microfibril orientation thickness
PMP (复合胞间层）: pectin and lignin; random network;
< 1%
S1 (次生1层）: Cellulose & hemi-, few lignin; θ= 50-70o;
~ 10%
S2(次生2层）: Cellulose & hemi-, few lignin; θ= 10-30o;
~ 70-90%
S3(次生3层）: Cellulose & hemi-, few lignin; θ= 60-90o;
< 10%
W(瘤层）: only occur in a few tree species
Ⅲ. Cell wall sculpturing
Wood cells that function primarily in the storage and
conduction of food materials are known as parenchyma.
These cells typically form thin secondary walls and are the
last to remain functional prior to heartwoods formation.
Other kinds of cells, in contrast, serve principally as
avenues of conduction in the living tree; these often form
thick secondary walls and thus are important in providing
mechanical support to stems in which they occur.
1. Pit（纹孔）
3-1 Definition
Regions on the cell wall in which the secondary portion of
the wall is lacking are known as pits.
3-2 Function
Pits serve as the conduction avenues between cell lumens.
3-3 Pit types
— simple pits（单纹孔）:
occured on the thin walls of parenchyma cells.
— bordered pits（具缘纹孔 ）:
occured on the thick walls of prosenchyma cells.
3-4 Pit pairs（纹孔对）
simple pit pairs（单纹孔对）：
bordered pit pairs（具缘纹孔对）：
half-bordered pit pairs（半具缘纹孔对）：
3-5 Structure of bordered pit pairs
a
d
e
a: pit ring
b: pit torus
f
c
b
a:
b:
c:
d:
e:
f:
g:
纹孔环
纹孔托
纹孔塞缘
纹孔缘
纹孔道
纹孔室
纹孔口
g
c: pit margo
Pit membrane
d: pit border
e: pit canal
纹孔塞
f: pit chamber
g: pit aperture
纹孔腔
inner aperture
outer aperture
Pit cavity
3-6 Effects of pits on wood properties
The pits dotting the cell wall cause local variation
in the normal S-1, S-2, S-3 microfibril orientation
discussed earlier.
This variation in microfibril orientation is known
to affect directional shrinkage properties of wood
and its mechanical strength.
2. Spiral thickening / 螺纹加厚
2-1 Definition
In some woods, formation of the S-3 layer is followed
by development of spirally arranged ridges of
microfibril bundles on the lumen side of the secondary
wall. These ridges are termed spiral thickenings.
2-2 Values
• Strengthening the wall structure
• Serving as a clue to wood identification
Spiral thickenings occur in cells of relatively few
softwoods and thus, when present, are a valuable clue to a
wood’s identity. In hardwoods, spiral thickening is relatively
common. While helically arranged thickenings may form
throughout the length of a cell, this feature in other cells may
be restricted to only the tops or center portions. As location
of thickenings is often consistent within a given species of
wood, this factor can also be of diagnostic significance.
Reflection and practice:
1. Functions of cellulose, hemicelluloses and lignin in cell wall
construction?
2. Microfibril structure of cellulose in cell wall?
3. Layering structure of cell wall?
4. Structure of Bordered pits?
5. The three kinds of pit pairs?
6. What is microfibril angle?
7. What is spiral thickenings?