1

Chapter 7 The minute structure of

porous wood / 阔叶树材微观构造

The wood formed by hardwoods is much different from that produced by softwoods. Softwoods have a uniform arrangement of a few cell types and therefore are often without a distinctive appearance. Hardwoods, on the other hand, are composed of widely varying proportions of markedly different kinds of cells and are thus often uniquely and even spectacularly figured. Because of the unique figure possessed by many hardwood species, such woods are widely used for furniture, paneling, and other decorative purposes.

2

Main differences between hardwood and softwood

Softwoods are composed of a few significant cell types-hardwoods of many.

Only hardwoods contain vessels, a structure composed of vessel elements.

Wide rays of some hardwoods contrast with the uniformly narrow rays of softwoods.

Straight radial rows of cells characterize softwoods; they are not found in hardwoods.

It is important to note that a summary of hardwood-softwood differences does not include any reference to the relative hardness of the wood produced. Many softwoods produce wood that is harder and denser than wood produced by some hardwoods.

3

Ⅰ. Longitudinal cells （轴向细胞）

(a) Fusiorm cambial initial

(b) Early-wood vessel element

(c) and (d) Late-wood vessel element with caudate ends ， on the opposite and on the same side respectively

(e) A strand of longitudinal parenchyma

(f) A fusiform parenchyma cell

(g) A vascular tracheid

(h) A vasicentric tracheid

(i) A fibrous cell

4

1. Vessel elements （导管分子）

Several differences exist between hardwood and softwood xylem, but the fundamental anatomical difference is that hardwoods contain specialized conducting cells called vessel elements. Vessel elements are generally much larger in diameter than other types of longitudinal cells. Note that vessel elements are shorter than hardwood and softwood fibers but larger in diameter. The short length of vessel elements is traceable to the fact that they often do not grow in length during the maturation process and may become even shorter than the cambial initials from which they were produced. Normally, a number of vessel elements link end-to-end along the grain to form long tube like structures known as vessels.

5

1-1 The shape of vessel and vessel element

1-2 Function of vessels

Vessels in hardwood serve as avenues of conduction in the living tree.

6

1-3 Vessel arrangement (distribution, combination) Ring-porous wood — 环孔材 Diffuse-porous wood — 散孔材 Semi-porous wood — 半环孔材 Radiant porous wood — 辐射孔材 Tangent porous wood — 切线孔材 Staggered porous wood — 交叉孔材

7

1-4 End-to-end connection of vessel elements

Common end walls of longitudinally linked vessel elements are, however, perforated by unrestricted holes. To facilitate discussion about this feature, names are given to the common vessel element end walls (perforation plates 穿孔板 ) and the holes in them (perforations 穿孔 ).

• Perforation plates: the end walls of vessel elements

Perforation plates invariably slope at an angle toward the radial. Therefore, it is better to examine the perforation plates on radial sections under a microscope.

8

• Perforation: the holes on perforation plate

It is interesting to note that perforations do not develop in a random fashion; instead they form in one of several definite patterns. Because of this, the nature of vessel perforations is often useful as an aid in the identification of hardwood timbers.

— Simple perforation / 单穿孔

— Scalariform perforation / 梯状穿孔

— Foraminate perforation (Ephedra like ) / 网状穿孔 ( 麻黄状穿孔）

9

1-5 Side-to-side connection of vessels

Numerous pairs of bordered pits provide lateral communication from vessel to vessel. Three type of intervessel pitting can be seen:

(a) alternate pitting

(b) opposite pitting

(c) scalariform pitting

As is the case with perforation plates, the shape and arrangement of vessel-to-vessel pitting is often consistent within a given species and can be of assistance in wood identification.

10

1-6 Connection between vessels and other cells

Vessels often occur adjacent to fiber tracheids, longitudinal and ray parenchyma, or other kinds of cells. Although fiber tracheids and vessels are sometimes not linked by pitting, other kinds of cells typically form pits where they contact vessel elements.

1-7 Tyloses （侵填体） Formation

outgrowths of parenchyma cells into the hollow lumens of vessels Appearance: bubble-like, lustrous, thin-walled

11

Implications

— Permeability

The permeability of wood with abundant tyloses will be poor, and so it is difficult to dry or to impregnate with decay –preventive or stabilizing chemicals..

— Durability

Usually, wood with abundant tyloses can resist the attack of organisms so its durability is good.

— Identification

The existence of tyloses sometime can be used as feature to distinguish wood species, e.g., white oak ( abundant) and red oak (relative lack).

12

2. Wood fiber （木纤维）

The term fiber is often used in a general way to all wood cells isolated in pulping processes. However, in the context of wood morphology, the term wood fiber refers to a specific cell type. Thus libriform fibers, or fiber tracheids as they are more properly called, are long, tapered, and usually thick-walled cells of hardwood xylem. A casual look suggests a great similarity to the longitudinal tracheids of softwoods, but closer examination reveals several significant differences.Hardwood fibers are considerably shorter than softwood tracheids. The softwood tracheids average 3-4 mm in length; hardwood fibers, in contrast, have an average length of less than 1 mm. That fact explains why softwood tracheids are often preferred as raw material for paper manufacture. Fiber length is an important determinant of paper strength.

13

2-1 Shape of wood fibers

• small and long (about 1mm in length, 20-30um in diameter)• tapered to ends• thick-walled

2-2 Function

Mechanical support of the tree trunk and crown.

14

2-3 Types of wood fibers

• fiber tracheids with bordered pits

• libriform fibers with simple pits

2-4 Special fibers

• Septate wood fibers

• Gelatinous fibers a G-layer attached to Secondary wall

Horizontal barG-layer

bordered pit simple pit

15

3. Longitudinal parenchyma

图 7-11 轴向薄壁组织 -甘巴豆

3-1 Type and shape of Longitudinal parenchyma in hardwood

• Parenchyma strands

• Fusiform parenchyma cells

• Epithelial cells around gum canals

16

3-2 Differences from those in softwood

• Larger amount

Longitudinal parenchyma is relatively rare in softwood species (no more than 1-2% of the volume of those woods in which it does occur), the longitudinal form of parenchyma is often quite significant in hardwoods. Certain species of hardwoods contain no longitudinal parenchyma. Some hardwoods may, however, have up to 24% of their volume made up of longitudinal parenchyma cells; this figure may even exceed 50% for a few tropical hardwoods.

• Numerous distribution patterns

Discussed in chapter 3.

17

3-3 Gum canals in hardwood

• Gum canals occur in a few hardwoods and are similar to resin canals of softwoods.

• The hardwood canals are sometimes lined with parenchyma type epithelial cells. Normal gum canals can extend along the grain or be included in the wood rays, but the two types seldom present in the same wood.

• Traumatic gum canals can also appear in hardwoods, as shown in the figure.

18

Ⅱ. Rays （射线） As listed in the summary of hardwood-softwood differences, hardwood rays range in width tangentially from 1 to 30 or more cells. Softwood rays in comparison are generally one or, rarely, two cells in width. Also unlike softwoods, the cells of hardwood rays are all of the parenchyma type (although two distinct types of ray parenchyma are formed). Hardwoods characterized by very large rays, such as oak, exhibit distinctive ray patterns in both tangential and radial faces; such rays often add to a wood’s aesthetic appeal.

19

1. Types of ray cells

Although all ray cells are of the parenchyma type, there are, nonetheless, different types of hardwood ray cells. The difference is in cell shape or configuration.

a. Procumbent cells

b. Upright cells

c. Squared cells

20

2. Size of rays

Size of rays is usually determined by their width on tangential section.• Small rays: W < 0.05mm, not be seen with naked eyes.• Middle rays: W = 0.05 ~ 0.2mm, be seen with naked eyes.• Large rays: W > 0.2mm, evidently be seen with naked eyes.

3. Types of rays

• Divided by width — Uniseriate rays （单列木射线） — Multiseriate rays （多列木射线） — Aggregate rays （聚合木射线） — Oak-tape rays （栎型木射线）

21

• Divided by the cell types in the rays

— Homogeneous rays （同形木射线）： only procumbent cells

a. Uniseriate homogeneous rays

b. Multiseriate homogeneous rays

22

— Heterogeneous rays （异形木射线） : procumbent and upright cells

a. Uniseriate heterogeneous rays

b. Type I rays

c. Type raysⅡ

d. Type raysⅢ

a b

c d

23

4. Storied structure of rays

In some hardwood species, the rays tend to be arranged into definite, tangentially oriented tiers. In these woods, rays in each layer are roughly the same height, and all begin and end at about the same levels along the grain. Such woods are said to have storied rays, and they often exhibit a readily visible banded pattern on tangential surface. A storied cell arrangement is not restricted to ray cells. Almost any type of hardwood cell can occur in storied arrangement, and the resulting pattern is often similar to that produced by storied rays. This pattern will show on both tangential and radial surfaces, while that from storied rays will be seen only on the tangential. Storing of elements is primarily of interest for wood identification.

24

5. Inclusions in ray cells

Inclusions in ray cells can be crystals, silica, and numerous amorphous materials of complex chemical nature, including gums, resins, oil, latex, coloring matter and alkaloids.

a. Oil cells

b. Crystalliferous cells

c. Cells with nucleus

d. Sheath cells

a b c d

25

Reflection and practice:

1. The common four type wood tissues of hardwoods?

2. Shapes of vessel elements in earlywood and latewood of ring porous woods?

3. Patterns of vessel arrangement?

4. Patterns of perforation on perforation plate of vessels?

5. What is perforation plate?

6. Pitting patterns on side walls connecting vessels?

7. Distribution patterns of longitudinal parenchyma?

8. Main differences between softwood and hardwood?