Study of Wood formation in Dicot Stem

Wood formation is a complex biological process, involving five major developmental

steps, including

(1) cell division from a secondary meristem called the vascular cambium,

(2) cell expansion (cell elongation and radial enlargement),

(3) secondary cell wall deposition,

(4) programmed cell death, and

(5) heartwood formation.

Wood cells in the stems of trees originate through division of the vascular cambium
cells. This division can occur in one or two directions either parallel to the cambium ring or
perpendicular to the ring. The one which occur parallel to the ring is called Periclinal and
results in the formation of a pair of cells, a new cambium cell and either a new xylem or
phloem cell. An Anticlinal division is division perpendicular to the ring and creates a pair of
vascular cambium cells that provide for increasing the circumference of the cambium as the
tree stem enlarges. The two types of division can take place in the same cambium cells, but
do not occur simultaneously.

A highly idealised vascular cambium dividing periclinally to form two new cells, one
of the cell become a new cambium cells which is formed on the outside of the cambium to
become the bark cell and the other towards the inside which forms the wood cell. Within a
few days of formations of the two new wood cells undergoes a sequence of changes involved
in the maturation process. The shape of the cell changes, it increases in diameter and most
cells elongate, and all the cell enlarges. A newly formed wood cell consists of a thin
membrane like wall called primary wall and a fluid filled centre. A secondary wall is added
to the inner surface of the delicate primary wall, once the new cell reaches the full size and
shape.

The secondary wall is constricted using macromolecules (cellulose, hemicelluloses and lignin)
that are synthesized from biopolymer found in the fluid-filled centre of the developing cell.
Progressive layers are added to the cell wall from the inside. Eventually the fluid filled filling
is expanded and the cell has a thickened, rigid wall and a hollow centre. Cell formation and
maturation continues during the growing season. With the vascular cambium producing new
wood and bark cell as rapidly as the condition allow.

Early-wood cells are formed first during the early spring and those formed later are the
latewood cells. Fig.1 illustrates two visible different regions in a tree stem disk. The outer
ring is lighter in colour and is called the sapwood and the inner darker region is called
heartwood. In twigs and very small young trees the entire xylem (wood) portion is involved
in sap conduction upwards and this wood portion is f named as Sapwood. As the tree ages the
entire sapwood portion is not as needed to transport fluids to the leaves, so at the centre of the
tree, the cells cease, hence their nutrients and eventually die. This transformation from living
conducting cell tissues to empty dead cell is responsible for cells being converted into
heartwood. Heartwood is usually darker in colour than sapwood. Which is because of some
extractives in the cell wall of heartwood, but this is not true for all the species of wood that
the heartwood will be darker in colour.

Heartwood
Sapwood
Cambium layer

Phloem

Bark

Fusiform initials and Ray initials

The vascular cambium has two types of cells known as fusiform initials and ray
initials. Fusiform cells are elongated and ray cells are rectangular in shape.

Fusiform initials Ray initials

Fusiform initials are elongated cells Ray initials are rectangular in shape and are
and are arranged as bricks. shorter cells.

Fusiform initials cell give rise to These ray initials help in formation of
secondary xylem and phloem. phloem and xylem rays.

Secondary xylem is a wood portion Ray initials are cells that remain in the
as it is towards the inside and one meristematic region after cellular division
formed outside the cambium is and they continue to make further more cells
secondary phloem. This is what and form rays as because when growth
happens when fusiform initials continues cells move away from vascular
divide. cambium

It forms a vertical system of These form the horizontal system that stores
secondary xylem and phloem. food and water and helps in translocation.
 Anticlinal and Periclinal division

Wood anatomy and its importance

Introduction:
Wood is a porous and fibrous structural tissue found in the stems and roots
of trees and other woody plants. Wood is sometimes defined as only the
secondary xylem in the stems of trees. The branch deals with study of
internal structure of an organism or object is known as anatomy.

Wood anatomy
Wood anatomy deals with the internal structure of wood including wood
formation and wood properties.

Importance and scope of wood anatomy

 Wood anatomical studies helps in correct identification of a timber,

 Assess its quality and suitability for diverse end uses. It is the
fundamental basis for all timber studies,
 Its direct importance is in the field of wood identification, wood
utilisation, paper and pulp, taxonomy, paleobotany etc.
 It also supplements the researcher in field of silviculture and genetic
tree improvement.
 The internal structure of a species is highly specific just like thumb
impression in human by way of which exact identity of a timber can
be established. Peoples are using timber since drawn of civilization
and they are able to recognise some of the common timbers like teak,
sal, sissoo, deodar, babool etc from their general appearance and
physical properties like colour, weight, odour etc. But this procedure
is not easy or nor practicable when hundreds of timber are involved.
The physical features are not always reliable as they may vary within
same timber or different timber appearance can be made similar by
artificial treatment. The best identity of timber is only from
anatomical structure. This study leads to proper identification of
wood which in turn play an important role in efficient wood
utilization. Example, Indian fir and spruce were found suitable for
aircraft and ship buildings. The studies of the tissues associated with
gum and resin production has lead in development better and more
efficient method of taping.
 These studies have also lead to find out reasons for seasoning defects
like warping twisting etc. These also help in removing the sawing
defects, application of wood preservation i.e. penetrability and
treatability of wood.
 The identification also lead in solving the special problem related
to currency note, revenue paper, bill document etc.
 Wood anatomical studies also plays a vital role in determining pulp
sheet properties like flexibility, tensile strength, tear, printability, etc
of different raw materials.
 Wood anatomical studies have been found to be wide significance in
archaeological studies. It helps to about type of vegetation, climate,
prevailing in older times through studies of growth rings.
 The effect of thinning, pruning, spacing, site quality and other
silviculture practices on wood characteristics is also studied under
wood anatomy.