The minute structure of coniferous woods

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Chapter 6

• The minute structure of coniferous woods
• ?????????

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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.
Fig. 22
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• Topical highlights
• I. Longitudinal coniferous tracheids
• ???????
• ?.Parenchyma in coniferous woods
• ???????
• ?.Transversely oriented cells
• ????
• ?.Crystalliferous wood elements in softwoods
• ?????????

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?. 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.
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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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5-2 Spiral thickenings Spiral thickenings
are present as a constant feature in the
longitudinal and ray tracheids in
Douglas-fir(???), Pacific yew(?????)and Torreya
spp(??). They may also occasionally occur in
other woods.
?6-6 ?????????
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• 6. Longitudinal strand tracheids(????)
• 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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?. Parenchyma in coniferous woods / ???????
There are three types of parenchyma in softwoods,
namely, longitudinal parenchyma(??????),
epithelial parenchyma(??????) and ray
parenchyma(??????). 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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?6-7 ?????????????????
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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???) 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(??? ????) Banded
parenchyma(??????) Terminal parenchyma(???????)
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?6-8 ???-??
?6-9 ??-????
?6-10 ??-????
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2. Ray parenchyma (??????) Ray parenchyma
cells are discussed in detail in the section on
rays. 3. Epithelial parenchyma and resin
canals(??????????)
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 (??)
• Picea(???)
• Cathaya(???)
• Larix(????)
• Pseudotsuga(???)
• Ketereeria(???) ??????
• Traumatic resin canals
• Occur only when the tree was injured.

Pinaceae
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• ?. Transversely oriented cells(????)
• 1. Type of transverse cells in softwoods
• ray parenchyma cells(??????)
• ray tracheids(????)
• epithelial cells(????)
• 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 parenchyma and the longitudinal tracheids (in
  earlywood) are called cross fields.
• Cross field pits

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(No Transcript)
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C
A
B
?6-16 ????? A ??? B ??? C ??? D ??? E ???
D
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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?6-17
?????? A. ?????????? B.????????????
C.??????????
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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 ?????)
rays with normal transverse resin canals
(fusiform rays ??????) 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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?. 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.
????-??
????-???
?????-???
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• Reflection and practice
• Cell types in softwoods?
• Shape and size of longitudinal tracheids?
• What is called cross field?
• The five types of cross field pits?
• How to distinguish hard pine and soft pine?