2. A Presentation by
Dr. N. Sannigrahi, Associate Professor,
Nistarini College, Purulia (W.B) India
3. INTRODUCTION
Wood, technically speaking , refers to the secondary xylem derived from the
consequence of secondary growth by the activity of vascular cambium as
found in the woody trees of gymnosperms and angiosperms across the
different phytogeographical regions of the world. But to a lay man, wood is
any parts of the plant used for fuel, constriction, support and many other
domestic activities along with some large scale industrial attributes. Wood
should not be confused with timber because it should be stated that all
timbers are wood but all wood are not timber as any plant part deserved to be
timber must possess some qualities for being qualified as timber.
Wood is a complex biological structure , a suitable chemistry and cell types
join together to serve the needs of a living plant. With the passage of time,
more and more secondary activity of the plants enable to add more and more
secondary xylem and this basically build up the structure and developmental
pattern of the wood to perform a lot of functions required for the same.
Wood is a complex structure and the arrangement of the different
components are the awesome pleasure to the plant anatomists for its various
texture, components, compactness and formation time.
4.
5. WOOD ELEMENTS
Wood although basically secondary xylem, still it deserves a wide diversity
as far as the elements are concerned. The following elements generally build
up the wood.
Secondary xylem has two systems of elements which differ as far as their
orientation is concerned. The vertical or longitudinal systems is known as
axial system consisting of vertical files of tracheary elements , fibers and
wood parenchyma.
The horizontal or transverse elements is known as radial system which
consists of xylem rays or wood rays.
The gymnosperms as well as angiosperms constitute the wood world but
both of them are little bit different to each other due to their internal beauty
and this has enabled the value of the wood property along with its
commercial attributes. The structure of wood is more simpler and more
homogenous in gymnosperms as compared to that of angiosperms due to the
absence of vessels( except in Gnetales) and presence of small amount of
axial parenchyma. In addition to that, the gymnosperms wood being almost
temperate in location differ to angiosperms due to their cosmopolitan
distribution.
6. Characters Gymnosperms Angiosperms
Vessel Wood lacks vessels They have vessels.
Axial parenchyma Present in less amount Axial parenchyma present.
Wood Popularly known as soft wood Wood is mainly hard wood.
Torus Bordered pits with torus is
common in Coniferales
Torus is not common
occurrence
Resin ducts Resin ducts are common in
Coniferales
Resin ducts are not
common in dicots
Rays fibers Rays are mostly uniseriate but
Libriform fibers are absent
Rays are multiseriate &
Libriform along with fiber
tracheids are present
Fenestriform Pits Pits are present window like Pit are not present
Crassulae and
trabeculae
Both are added features here. This type of thickenings are
absent.
7. AXILLARY ORIENTED ELEMENTS IN ANGIOSPERMIC WOOD
The vertical system in wood is constituted by the axillary elements and it
basically comprises of tracheary elements, vessels, libriform fibers, fibre-
tracheids and wood parenchyma.
VESSELS:
1.Only found in dicots as major water conducting elements of wood,
2. It is characterized by the presence of perforation plate at their end walls and
this perforation plate enables the vessel membrane joined to one another to
form a tube like structure (synctes) for the movement of cell sap,
3. The perforations plates are lateral or sub-terminal in position and perforated
areas are larger than the pits,
4.The partial hydrolysis of the cell wall generally forms the perforations plates
during the cytodifferenciation of the tracheary elements,
5. Areas surrounding the perforation plate is known as perforation rim,
6. Perforation plate with one large pore is called simple perforation but more
than one perforation plate is called multiple perforation plate,
7. Simple perforation probably derived from multiple perforation plate by
the loss of secondary thickenings,
8. As per arrangement of the perforation plates, they may be scalariform
8. Plates, reticulate plates , foraminate as per their arrangement of the pores,
9. Vessels with transverse end walls and simple perforations are considered to
be advanced.
TRACHEIDS
1. These are non-perforated cells in which only one bordered pits airs are
found in the areas of contact between them,
2. Three types of tracheids- vessel tracheids, vasicentric tracheids and fibre
tracheids,
3. Vessel tracheids are fusiform in shape and arranged in axial strands which
are interconnected by bordered pits arranged in scalriform manner on the
anticlinal oblique end walls,
4. Walls of the vessel tracheids have helical thickenings,
5. Groups of vessel tracheids strands look like cluster of small vessels but do
not posses any perforation plates,
6. Vessels tracheids are commonly found between vessels and provide
pathway for transport,
7. They are least susceptible to embolism as they are narrow and connected
with other neighboring cells,
8. Vasicentric tracheids are shorter and located around the periphery of wider
vessels and are in contact with other parenchyma cells with numerous pits.
9.
10. They act as armor of the vessel walls , damping shocks or vibrations that
could break adhesive bonds between the gap solution and the walls,
10. Fibre tracheids- Intermediate between tracheids and fibers, longer than
vessels and tracheids due to their appreciable growth of their tips during
differentiation, have thin walls, bordered pits and provide mechanical
support.
FIBRES
i. Two types of fibers are found in the axillary components of dicot wood-
living fibers and libriform fibers,
ii. Living fibers are septate, contain protoplast , starch grains and can not
help in transport of water in apoplastic system,
iii. To perform mechanical support,
iv. LIBRIFORM FIBERS- Long contain thick walls and simple pits;
resemble to phloem fibers found in the species having short cambial
fusiform initials. Sometimes, they are found in abundance so that the
tracheary elements and parenchyma are embedded in the cellular matrix
of libriform fibers.
11.
12. WOOD PARENCHYMA
Parenchyma is a simple permanent tissue and wood possess parenchyma of
two types- Axial Parenchyma and Ray parenchyma. The former develops from
long fusiform initials of vascular cambium arranged parallel to the axis of the
stem whereas the later initiates from short ray initials and is arranged radially.
AXIAL PARENCHYMA
The axial parenchyma possess the following features-
i. The cells are as long as fusiform initials or sometimes shorter,
ii. The axial parenchyma cells do not divide transversely form fusiform
parenchyma,
iii. Mature cells retain protoplast for many years,
iv. The different species have considerable variations from less quantity to
more,
v. Absence of axial parenchyma is considered to be more primitive feature,
vi. Axial parenchyma of two types- apotracheal type & paratracheal type
vii. ,; When the distribution of parenchyma is of independent of vessels called
apotracheal and parenchyma with close association of vessels called
paratracheal type.
viii. Both of them contribute in the formation of wood
13. APOTRACHEAL
PARENCHYMA
PARATRACHEAL
PARENCHYMA
The axial parenchyma of secondary
xylem is situated independent of
vessel
The axial parenchyma of secondary
xylem is distinctly associated with
the vessel.
The apotracheal parenchyma of two
types- Diffuse parenchyma ,
Banded parenchyma & Boundary
Parenchyma.
The paratracheal parenchyma may
be scanty, vasicentric or Aliform.
The parenchyma of diffuse
apotracheal appear as single cell or
small uniseriate band throughout
the growth ring( Quercus)
Paratracheal scanty parenchyma
cells do not form continuous sheath
surrounding the vessel (Acer).
Banded parenchyma appear as
concentric rings (Hicoria)
Vesicentric parenchyma cells form
a continuous sheath around the
vessel of different diameter (
Tamarix)
14. Types Features
Diffuse or Scattered When the apotracheal parenchyma is present in
the form of isolated cells and is scattered
among the fibers, this shapes appear.
Banded or
Metatracheal
Parenchyma
When the axial parenchyma is present in the
form of concentric bands
Terminal Parenchyma When the single apotracheal parenchyma is
arranged in more or less continuous layers at
the end of the growth ring with variation in
their width.
Initial Parenchyma When the above parenchyma is formed at the
beginning of the growth ring
15. RADIALLY ORIENTED ELEMENTS
i. Radially oriented elements include sheets of parenchymatous cells which
are arranged at right angles to the longitudinal axis of the plants. These are
called wood rays or xylem rays.
ii. The increase of the concentric growth ring is accompanied with the rays,
iii. There is a wide variation in the cellular composition of the rays. When the
ray cell is one cell wide, it is called uniseriate rays , when two cells wide
called biseriate and more than two cells wide is called multiseriate ,
iv. Uniseriate & multiseriate rays can be present in the same species,
v. Very often in some species, aggregate rays are also present consists of a
group of small & narrow rays in a single large rays,
vi. If all the cells are procumbent, the ray is homogenous and it is mostly
found in the arbores cent genera of temperate zones.
vii. In common tropical trees, medullary rays consists of radially elongated
cells (Procumbent cells) and upright or in square or vertically elongated
cells (erect cells), it is called heterogeneous cells.
viii. Ray cells are two types – Contact ray cells and Isolated ray cells,
ix. Contact ray cells have pit contacts with neighboring tracheary elements
and can release solutes directly to these elements
16. The pits of these rays are relatively large and simple,
x. Isolated ray cells do not have pit contacts with tracheary elements. They
exchange solutes with other living cells symplastically , but can not release
them directly into tracheary elements.
xi. Upright cells are always of contact types but procumbent cells may be either
the contact or isolated type.
WOOD PROPERTIES
Wood is the most important for the different household and industrial purposes
due to its different physical, chemical , mechanical along with its biological
properties as stated below:
PHYSICAL PROPERTIES
The characteristics that speaks about wood is nothing but the physical
properties of the wood as stated below.
COLOR- Color of the wood plays a very important role in this regard and the
color varies from dark brown to other color due to the deposition of the
secondary chemicals as deposited thereafter. The color plays an important
component for the physical beauty of the wood.
17. LUSTRE: Lustre refers to the tendency to the reflection of light. After color,
lustre deserve importance in this regard and different woods have different
lustres.
ODOUR & TASTE: The wood possess different types of secondary
chemicals and it determines the odor and taste of the wood. Sandal wood has
typical aroma due to the presence of sandal oil but other timbers may have
tobacco like odor & taste.
DENSITY : The density of the wood refers to the mass per unit volume. The
density refers to the weight of the wood sample. Lighter the density, least the
weight or vice versa. If density or specific gravity below .36, it is called light
wood. If it is 0.35-0.5 , it is called light.
HARDNESS: It refers to the strength of the wood or the resistance or capacity
of the wood which can strong after a long time even being affected by several
other factors.
MECHANICAL PROPERTIES: It is the capability of the wood to withstand
external forces like strength, resistivity, elasticity, durability etc. Compression
of the wood depends upon the directions of the applied external forces.
Tension, bending etc are some of the properties deserve mentioning in this
regard.
18. DIFFERENT TYPES OF WOOD
Wood being formed from the secondary xylem during the secondary growth ,
the nature of the wood, its texture, quality, longevity depends upon the
structural, components of the constituent elements along with their time of
formation, effect of seasons etc. Depending of all the above features, wood may
be of the following features-
a. Early Wood (Summer )& Late wood( Autumn ): The marked variations in
the cambial activities due to different seasons in the temperate climate,
two different types of cambial activity are noticed by the change of
temperature . In each year, a spring summer season is followed by cold
winter or autumn season. The wood formed during winter is called late
wood or autumn wood and the wood is formed during the spring season is
called early wood or summer wood. The two rings of wood formed in a
year make the growth ring. In other words, growth ring can be
distinguished into spring & autumn wood on the basis of the difference in
shape, structure and distribution of homogenous elements in two regions.
b. In spring time , cambial activity is increased due to longer duration of sun
shine accompanied with the increase in temperature , maximum vegetative
activity and more hormonal supply due to newly formed leaves.
19.
20. c. In early wood, the secondary xylem has much wider vessels which are
relatively thinner in the late wood. In spring, the cambium divides rapidly
to more amount of xylem with light color vessels and low density.
d. In winter, temperature is low, low relative humidity and shorter duration of
light which shows down the cambial activity. The autumn is narrow in
diameter, comparatively thick walled , darker in color and higher in
density with high strength and density.
SAP WOOD & HEART WOOD
In most of the woody plants, the outer portion of the secondary xylem is
lighter in color than the inner dark colored zone. The outer light colored
part of the wood is called sap wood or alburnum and the central dark
colored part is called hear wood or Duramen.
i. Sap wood contains living cells and is involved in the conduction of
water, storage of starch and fats. Due to presence of this type of
materials, sap wood is prone to be attacked by insects and degredative
with least durability. The timber value is also decreased proportionally.
ii. The inner dark zone is called heart wood or Duramen , composed of
thick lignified dead cells impregnated with secondary metabolites like
resins, oils, gums and other colored substances.
21.
22. iv. All wood usually starts from sap wood in young trees but as the tree gets
older, its trunk increases and heart wood gradually forms from sap wood. A
gradual change involves the disintegration of protoplasts, reduction of water
content and removal of water content and reserve food matters from the cells.
Cells become heavily lignified and develop colored and aromatic substances
due to the oxidation and polymerization of phenols followed by the
disappearance of starch.
v. The zone of wood showing this changes is known as transition. The living
cells but senescent cells in the transition zone undergoes metabolic activity b
during which Flavonoids arte produced. In angiosperms, the vessels are
completely occluded by the formation of tylosis. All the changes make the
transformation from sap wood to heart wood.
There is a sharp distinction of colors of sap wood and heart wood in Pinus,
Dalbergia & Albizzia but gradual changes in Shorea robusta, Adina cordifolia
but no distinction of color at all in Abies pindraw and Picea smithiana .
vi. Sap wood performs physiological function but heart wood performs
mechanical function.
23. SAP WOOD HEART WOOD
It is Also known as alburnum and is
situated on the peripheral side of the
secondary xylem.
It is known as Duramen and is
situated on the inner side of the
secondary xylem
It is lighter in color in contrast to
heart wood and consists of active
secondary xylem,
It is darker in color in contrast to sap
wood and consists of inactive
primary and secondary xylem
It is softer in texture and later
formed wood than hear wood,
It is harder and early formed wood
than sap wood
Tylosis is usually absent here Tylosis is frequently found in the
vessels and blocks the lumen
Translocation and storage of food
occurs through vessels and xylem
parenchyma respectively
Vessels are non-functional due to
accumulation of gums, resins,
tannins etc of secondary metabolites
and xylem becomes highly lignified
The sap wood gradually transformed
into heart wood.
Heartwood is more durable and least
resistant to decay than sap wood.
24. RING POROUS & DIFFUSE POROUS WOOD
The presence of vessels in angiosperms makes the difference with the
gymnosperms and the vessels of angiosperms make the wood porous whereas
gymnosperms as non-porous. The porous wood is called hard wood and non-poros
wood is called soft wood.
The environmental conditions and the age of the plant influence markedly the
arrangement of vessels and on the basis of the arrangement of the vessels, it is
classified as ring porous wood and diffuse porous wood.
The vessels may be of different diameter. The wood which contains vessels are
arranged in the form of ring called ring porous wood . The vessels are larger and
are marked as the onset of the season like Morus alba, Toona ciliata but when the
vessels are more or less equal in diameter and are uniformly distributed throughout
the wood, it is called diffuse porous wood as in Acer sp. & Poplus alba, Acacia
cyanophylla and others.
Ring porous wood are advanced over the diffuse porous wood . Length of vessels
in ring porous wood is longer than the diffuse porous wood , it’s development is
rapid and sudden while diffuse porous wood is slow.
25.
26. RING POROUS WOOD DIFFUSE POROUS WOOD
The vessels are of different diameter
and not uniformly distributed
throughout the wood.
Vessels are more or less equal in
diameter and uniformly distributed
throughout the wood.
Vessels with wide & smaller diameter
are formed early and later part of the
growth season.
Vessels with more or less equal diameter
are formed throughout the growth ring.
Vessels with wide diameter of early
wood and vessels of smaller diameter of
autumn wood are distinguishable.
Vessels of early wood and late wood are
indistinguishable.
The development of vessel is sudden
And rapid.
The development of vessel is slow.
The vessels are longer in length than
those of diffuse porous wood.
Vessels are shorter in length than those
of ring porous wood.
The rate of transport of water in plant
with ring porous wood is ten times faster
than those of diffuse porous wood.
The rate of transport of water in plants
with diffuse porous wood is slower than
those with ring porous wood.
27. REACTION WOOD
Due to unusual mechanical pressure due to stress and strain conditions of the
environmental impact, some special type of wood formed in woody plants and
they are different in structure and properties as far as normal wood is concerned.
It is mostly found in branches and leaning stems. It is generally found in response
to non-optical orientation of the stem or branch caused by prevailing stress like
winds. Snow, slopes, loads to even asymmetric shape of the crown. As this wood
generally forms as a part of the reaction corresponding to stress of the entrusted
environment, this type of wood is called reaction wood.
For example, when a plant is brought out of its equilibrium position and inclined
or placed horizontal, a longitudinal growth on the lower side is not sufficient to
make a bend upward. The right orientation is achieved by the formation of
reaction wood. It is wider than the normal increments so that the cross section is
very often asymmetric or elliptical.
Two types of reaction wood are generally found-Tension wood and compression
wood.
TENSION WOOD
The Tension wood posses the following properties as stated below:
i. Mostly found in the dicotyledonous angiosperms on the upper side of the
branches of the stem where the tissues are held in tension.
28.
29. ii. Characterized by gelatinous fibers which have thick, highly gelatinous layer in
the cell wall.
iii. Vessels are less in number with reduction of width.
The amount of lignin is less in tension wood in comparison to normal wood but6
the amount of cellulose is high.
iv. The most outstanding feature of the tension wood is the presence of a thick
inner cell wall layer that consists of highly crystalline cellulose,
v. During maturation, the tension wood fibers shrink strong longitudinal direction
, thereby creating a very strong tensile stress to pull the leaning stem upright,
It is found that the low concentration of IAA promotes the development of tension
wood in dicotyledons,
On the basis of the arrangement of gelatinous fibers, two types of tension wood
classified- Compact tension wood & diffuse tension wood. In the compact tension
wood, the fibers form continuous region (Acer) whereas in the Diffuse tension
wood, fibers are scattered in single or in groups among the normal fibers (
Acacia).
30. COMPRESSION WOOD
i. Mostly found in gymnosperms especially on the lower side of the stem
of Conifers,
ii. It is found by the activity of the vascular cambium,
iii. Wood is characterized by highly lignified cell wall and eccentric growth
ring,
iv. The tracheids are shorter, round or with some intercellular spaces which
are filled with an amorphous water absorbing substances,
v. It is 15-40% heavier than the normal wood for which it is more brittle
and capable of unusual high and irregular shrinkage . Transverse
shrinkage is less
vi. Cell wall contains less cellulose but more lignin,
vii. Helical cavities are another feature of this wood and are associated with
some physical properties. As a result of these changes, compression
wood cells can expand longitudinally during the maturation of the cell
to perform the function of pushing up the shoot or stem.
viii. It is generally induced by different phytohormones like auxins, GA,
Ethylene but mechanisms yet to be explored
31. PERIDERM POLYDERM
It is usually formed on the aerial stems
and roots
Usually formed on the underground stems
and roots
Composed of three layers of cells-
phellem , phellogen and phelloderm
collectively called periderm.
Composed of alternating layers of cells
composed of uniseriate endodermal cells
and multiseriate parenchyma collectively
called polyderm.
Suberin and sometimes lignin and wax
are deposited.
Suberin is deposited on endodermis like
cells
Casparian strips are not formed. Casparian strips are formed before
suberization.
The phellogen originates either at
epidermis, hypodermis, cortex or phloem.
The phellogen originates at the pericycle.
The phellogen forms centrifugally
phellem and centripetally phelloderm
cells
The phellogen centrifugally forms
polyderm.
Phellem & phelloderm are respectively
concerned with protection and storage
The outermost dead cells and non--
suberized cells are for protection and
storage
32. CONCLUSION
Wood has been considered as one of the important materials for the different
purposes of the civilization starting from fuel initially to the construction
materials along with other purposes with the passage of time. Wood being
secondary xylem performs manifold attributes. The wood is basically the
outcome of the vascular cambium and this cambium undergoes division to
add more and more secondary tissues with the passage of the growth rings of
the plants. The nature of the different elements of the secondary xylem along
with the deposition of the secondary materials bring the changes of the
physical and mechanical attributes of the plants. In addition to these features,
the wood also conveys the information of the climate of the areas which it has
been experienced in course of their life time.
Dendrochronology(determination of the age of the plant by counting the
number of annual rings) & Clime-dendrochronology are the two important
fields of study that draw attention to the anatomists and climatologists to
explore the climate related data of any particular geographical zones. Thus,
wood and wood science has both the theoretical as well as practical
approaches in this regard.
33. References:
1. Google for images,
2. Different open sources of information of WebPages
3. Plant Anatomy- A. Fahn
4. Plant anatomy- B.P. Pandey,
5. A textbook of Botany (Vol. II) Ghosh, Bhattacharya, Hait
6. Plant anatomy- Pijush Roy,
5. Anatmy of Angiosperms- Mishra & Dash
DISCLAIMER:
This presentation has been made to enrich open source of
learning without any financial interest. The presenter
acknowledges Google for images and other open sources of
information to develop this PPT.
