Transcript Chapter 6 The minute structure of coniferous woods 针叶树材的微观结构

Chapter 6
The minute structure of coniferous woods
针叶树材的微观结构
Softwoods have traditionally been the mainstay of the wood
products industry, and these woods continue to be extremely
important today. The homogenous, straight-grained, and
lightweight softwood is preferred for construction lumber and
plywood. Tall, straight-boled softwoods are used for poles and
pilings. Because they are typically composed of long fibers,
softwoods are also a premium raw material in the manufacture of
strong papers. A knowledge of the physical nature of softwood
xylem is basic to an understanding of wood and wood products. The
structural characteristics of this important group of woods are
examined in this chapter.
The xylem of softwoods is quite simple. Most species have no more
than four or five different kinds of wood cells, and only one or two
of these occur in appreciable numbers. Because of this simplicity
and uniformity of structure, softwoods tend to be similar in
appearance.
2
Topical highlights
I. Longitudinal coniferous tracheids
针叶材纵向管胞
Ⅱ.Parenchyma in coniferous woods
针叶材薄壁组织
Ⅲ.Transversely oriented cells
横向细胞
Ⅳ.Crystalliferous wood elements in softwoods
针叶材中的含晶细胞
3
Ⅰ. Longitudinal coniferous tracheids
The longitudinal coniferous
tracheids are relatively long, four- to
six-sided, prismatic cells with closed
ends. They are the principal type of
cell in the wood of all softwoods.
4
1. Shape: 4-6 sided, prismatic cells with closed ends
2. Size: 3-5 mm in length, 30-45um in diameter
3. Volume proportion: 90-94 %
4. Arrangement: regularly arranged in radial rows
5
5. Markings of longitudinal tracheids
The walls of the longitudinal tracheids are
commonly marked with pits and in a few conifer
species also with spiral thickenings on the innerwall surface
5-1 Pits
The pits in the longitudinal tracheids fall into three
categories:
•
those that belong to pit pairs between the
adjacent longitudinal tracheids
•
those that are a part of pit pairs providing
communication between the longitudinal tracheid
and ray parenchyma cells in contact with it
•
those that belong to pit pairs that connect the
longitudinal tracheids with the ray tracheids,
when these are present in the ray.
6
5-2 Spiral thickenings
Spiral thickenings are present
as a constant feature in the
longitudinal and ray tracheids in
Douglas-fir（花旗松）, Pacific
yew（太平洋紫杉）and Torreya
spp（榧树）. They may also
occasionally occur in other
woods.
图6-6 银杉管胞壁螺纹加厚
7
6. Longitudinal strand tracheids（索状管胞）
The term strand tracheids are applied to the short thickwalled cells arranged in longitudinal strings or strands. These
cells differ from the longitudinal tracheids in being shorter and
in possessing end walls, one or both of which are at right angles
to the longitudinal walls.
Strand tracheids may be regarded as transitional element
between longitudinal tracheids and epithelial or longitudinal
parenchyma.
• Shape:
• Location: in the vicinity of the longitudinal resin canals
• Value: a diagnostic feature for some species
8
Ⅱ. Parenchyma in coniferous woods / 针叶材薄壁组织
There are three types of parenchyma in softwoods, namely,
longitudinal parenchyma(轴向薄壁组织）, epithelial parenchyma
（伴生薄壁组织） and ray parenchyma（射线薄壁组织）.
1. Longitudinal parenchyma
1-1 Appearance of longitudinal parenchyma
The longitudinal parenchyma in softwoods occurs in the form of
strands extending along the grain. Each strand arises by the further
division of a daughter cell formed by division of a fusiform initial in
the cambium. When cut transversely, such a strand appears as a cell,
which is usually thinner-walled than the neighboring tracheids and
frequently contains extraneous materials, as shown below.
9
图6-7 竹柏横切面及径切面上的轴向薄壁细胞
10
1-2 Pits in the walls longitudinal parenchymatous cells
Pits in the walls of axial parenchyma when present are
invariably simple.
When viewed in the longitudinal sections, simple pits give the
appearance of beadlike (nodular结节的) thickenings on the
transverse walls of parenchyma cells.
11
1-3 Distribution patterns of axial parenchyma in softwood
Longitudinal parenchyma is never very abundant in
softwoods. However, it may be quite conspicuous because of the
presence of dark-colored inclusions in many of the cells. Usually,
distribution of longitudinal parenchyma in softwood can be
divided into three patterns.
— Diffuse parenchyma（星散型 薄壁组织）
— Banded parenchyma（带状薄壁组织）
— Terminal parenchyma（轮界型薄壁组织）
12
图6-8 星散型-翠柏
图6-9 带状-日本花柏
图6-10 轮界-藏南铁杉
13
2. Ray parenchyma （射线薄壁细胞）
Ray parenchyma cells are discussed in detail in the section on
rays.
3. Epithelial parenchyma and resin canals（伴生薄壁细胞和树脂道）
The epithelial cells are excreting, thinwalled parenchyma cells which surround
longitudinal and transverse (ray) resin
canals. The cavity of a canal is in reality
an intercellular space, i.e., a place where
immature longitudinal xylary cells pulled
apart, leaving a tubular cavity. The resin
canal as such is therefore not a wood
element but a cavity surrounded by thinwalled, parenchymatous cells (epithelial
cells).
14
• Genera with normal resin canals in China
Pinus (松属)；
Picea(云杉属)；
Cathaya(银杉属)；
Larix(落叶松属)
Pseudotsuga(黄杉属)；
Ketereeria(油杉属) ——无径向树脂道
Pinaceae
• Traumatic resin canals
Occur only when the tree was injured.
15
Ⅲ. Transversely oriented cells（横向细胞）
1. Type of transverse cells in softwoods
• ray parenchyma cells（射线薄壁细胞）
• ray tracheids（射线管胞）
• epithelial cells（伴生细胞）
1-1 ray parenchyma cells
• Cell features:
brick-like, thin-walled, with simple pits and beadlike end wall
•
•
Cross field:
The common-wall areas between cells of the ray
parenchyma and the longitudinal tracheids (in
earlywood) are called cross fields.
Cross field pits:
16
17
A
B
C
图6-16 交叉场纹孔
A 窗格型 B 杉木型 C 柏木型
D 云杉型 E 松木型
D
E
18
1-2 Ray tracheids
Ray tracheids occur normally in the Pinaceae and are constant
features of the woods of Pinus, Picea, Larix, Pseudotsuga, and Tsuga.
• Cell features
brick-like, thick walled, with boarded pits of the same type as those
found in the longitudinal tracheids, only smaller
• Location
Ray tracheids usually present on the upper and lower margins of the
ray, and not infrequently in the median portion as well. The low rays
of hard pines frequently consist wholly of ray tracheids.
• Tooth-like thickenings
The inner wall of ray tracheids in hard pines possesses irregular
tooth-like projections. Ray tracheids with these types of wall
irregularities are said to be dentate.
• Hard-pine & soft-pine
The tooth-like thickenings serve as an easy and positive means to
distingush hard pines (with) and soft pines(without).
19
图6-17 射线管胞类型
A. 射线管胞位于射线上缘 B.射线管胞内壁平滑至微锯齿 C.射线管胞内壁齿状加厚
20
2. Rays in softwoods
The wood- or xylary rays in softwoods consist entirely of transversely
oriented cells, which are arranged in ribbon-like structures extending
radially in the tree stem. The volume of ray tissues in softwoods is quite
small, ranging from about 5 to 9 percent
Coniferous woods may have either one or two types of rays:
— rays without resin canals (uniseriate rays 单列木射线)
— rays with normal transverse resin canals (fusiform rays 纺锤形木射线)
The inclusion of resin canals in the rays results in the formation of
fusiform rays, so-called because of their spindle shape when cut
transversely.
21
Ⅳ. Crystalliferous wood elements in softwoods
Crystal-bearing cells are rarely encountered in the softwoods.
When present, crystals occur in the form of exceedingly small
cubes, octahedra, or rectangular prisms, and are ordinarily
associated with parenchyma cells.
砖块状晶体-白皮松
星状晶体-银杏
柱状晶体-金钱松
22
Reflection and practice:
1. Cell types in softwoods?
2. Shape and size of longitudinal tracheids?
3. What is called cross field?
4. The five types of cross field pits?
5. How to distinguish hard pine and soft pine?
23