Biology Today — December 2017

Softwood and Hardwood
Softwood refers to the gymnosperm wood devoid of vessels. It is also called non-porous wood. It is mainly composed of tracheids
(90-95%) and fibres, though vascular rays constitute 5-10% of wood. It is easy to work with e.g., Cedrus, Pinus. Despite being
called ‘soft’, all of them are not soft.

Hardwood is the technical term for dicot wood which possesses abundant vessels. It is also called porous wood. In such wood,
the fibres are abundant while tracheid content is very low (<5%).
Ring porous
The wood
in which
vessels are
comparatively
broad in spring
wood and
quite narrow in
autumn wood
e.g., Dalbergia
sissoo.

Diffuse porous

The wood

where large

sized vessels

are distributed

throughout

spring wood

and autumn

wood e.g.,

Syzygium

cumini.

Two

types of

porous

wood

Ring porous wood is considered to be more advanced than diffuse porous wood as it provides better transition in case of high

requirements by the plant.

Activity of Cork Cambium

So as to prevent damage due to rupturing of outer ground tissues owing to formation of secondary vascular tissues and to increase

in girth, dicot stems give rise to cork cambium or phellogen in its outer cortical cells. In rare cases, it may arise from epidermis

e.g., teak, hypodermis e.g., pear or phloem parenchyma.

Cork cambium (phellogen)

(Few rows of living, active, narrow,

thin walled and roughly rectangular

cells, divide on both sides)

Epidermis

Cortex Cortex

Epidermis

Phelloderm

(Parenchymatous or

collenchymatous, cells

usually contain chloroplast)

Phellem

(Due to tannin, cork

appears brown in colour)

Cork cambium

Cuticle

Cortex

Phellem (cork)

(Suberised cells

towards outside)

Phellem

(Dead compactly arranged cells with

suberised cell wall and contain tannins)

Periderm

Sometimes, phellogen produces aerating pores instead of cork, which are referred to as lenticels.

Fig.: Activity of cork cambium : Formation of phellogen, phelloderm and phellem

Lenticels

These are aerating pores in the bark of plants that appear as raised scars containing oval or oblong depressions on the surface.

These occur only in woody trees either in scattered or longitudinal rows, produced beneath the stoma of epidermis.

Broken epidermis

Cork (phellem)

Cork cambium (phellogen)

Secondary cortex (phelloderm)

Fig.: T.S. through lenticel

Complementary cells

(loosely arranged, thin walled, suberised

or unsuberised complementary cells,

enclosing large intercellular spaces for

gaseous exchange. Check excessive

evaporation of water)

During winter, the lenticels get closed in temperate plants due to formation of compact closed cells over the complementary cells.