Chapter 6The minute structure of coniferous woods

针叶树材的微观结构

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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.

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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

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Ⅰ. Longitudinal coniferous tracheids

The longitudinal coniferous trac

heids are relatively long, four- to six-

sided, prismatic cells with closed end

s. They are the principal type of cell i

n the wood of all softwoods.

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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

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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 inner-wall 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.

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图 6-6 银杉管胞壁螺纹加厚

5-2 Spiral thickenings

Spiral thickenings are present

as a constant feature in the longi

tudinal and ray tracheids in Dou

glas-fir （花旗松） , Pacific yew

（太平洋紫杉） and Torreya spp

（榧树） . They may also occasio

nally occur in other woods.

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6. Longitudinal strand tracheids （索状管胞）

The term strand tracheids are applied to the short thick-walled 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

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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.

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图 6-7 竹柏横切面及径切面上的轴向薄壁细胞

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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.

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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）

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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

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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, thin-walled 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 thin-walled, parenchymatous cells (epithelial cells).

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• 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

• Traumatic resin canals Occur only when the tree was injured.

Pinaceae

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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 bead-

like end wall

• Cross field: The common-wall areas between cells of the ray p

arenchyma and the longitudinal tracheids (in earlywood) are called cross fields.

• Cross field pits:

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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

A B

D

C

E

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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).

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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

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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.

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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

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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?