5.4.2 Anatomy of Dicot Stem (Sunflower)

A transverse section of sunflower stem shows the following structures :

1. Epidermis:

It is single, outermost layer with multicellular outgrowth called trichomes.

A layer of cuticle is usually present towards the outer surface of epidermis.

2. Cortex:

It is situated below the epidermis and is usually differentiated into three regions namely, hypodermis, general cortex and endodermis.

 

i) Hypodermis:

It is situated just below the epidermis and is made of 3-5 layers of collenchymatous cells.

Intercellular spaces are absent.

ii) General cortex:

It is made up of several layers of large parenchymatous cells with intercellular spaces.

iii) Endodermis:

It is an innermost layer of cortex which is made up of barrel shaped cells.

It is also called starch sheath.

 

3. Stele:

It is the central core of tissues differentiated into pericycle, vascular- bundles and pith.

 

(a) Pericycle:

It is the outermost layer of vascular system situated between the endodermis and vascular bundles.

In sunflower it is multilayered and also called hard bast.

(b) Vascular bundles:

The vascular bundles are conjoint, collateral, open, and are arranged in a ring.

Each vascular bundle is composed of xylem, phloem and cambium.

Xylem is endarch and is made up of vessels, tracheids, xylem fibres and xylem parenchyma.

Phloem consists of sieve tubes, companion cells, phloem fibres and phloem parenchyma.

A strip of cambium is present between xylem and phloem.

(c) Pith:

It is situated in the centre of the young stem and is made up of large-sized parenchymatous cells with conspicuous intercellular spaces.

Anatomy of Monocot Stem

It differs from dicot.

Epidermis is without trichomes and the hypodermis is sclerenchymatous.

Vascular bundles are numerous and are scattered in ground tissue.

Each vascular bundle is surrounded by a sclerenchymatous bundle sheath.

Vascular bundles are conjoint collateral and closed (without cambium).

Xylem is endarch and shows lysigenous cavitiy.

Pith is absent.

Secondary growth is also absent.

Secondary growth:

In dicot stem, secondary growth begins with the formation of a continuous cambium ring.

The cambium present between the primary xylem and primary phloem of a vascular bundle is called intrafascicular cambium.

The cells of medullary rays adjoining these intrafascicular cambium strips become meristematic (regain the capacity to divide) and form the interfascicular cambium.

Thus a complete and continuous ring of vascular cambium is formed.

The cambium ring cuts off new cells, towards both the sides inner and outer.

The cells that are cut off towards pith(inner-side) mature into secondary xylem and cells that are cut off towards

periphery mature into secondary phloem.

Generally amount of secondary xylem is much more than the secondary phloem.

In temperate regions, due to the changes in climatic conditions there is formation of annual rings.

In spring, cambium produces large number of xylem elements and vessels with wider cavities, this is called spring wood or

early wood.

In winter, cambium is less active and produces few xylem elements that have narrow vessels, this is called autumn wood or

late wood.

The spring wood is lighter in colour and autumn wood is darker and has higher density.

The two kinds of woods that appear as concentric rings, constitute an annual ring.

In old trees, secondary xylem (wood) in the central region, does not conduct water and it becomes dark, due to deposition

of tannin, resins, oils, aromatic substances, etc.

These substances make the wood hard, durable and resistant to microbes and insects; this is called heart wood.

The peripheral region of the secondary xylem is lighter in colour and is known as sapwood.

Sap wood is involved in the conduction of water and minerals.

 

The outer cortical layers and epidermis get broken due to the activity of vascular cambium, therefore it is replaced by periderm.

In the cortex, certain cells regain the capacity to divide and corkcambium (phellogen) gets formed.

Cork cambium also cuts off cells on both the sides.

The outer cells differentiate into cork while the inner cells form secondary cortex.

Cork, cork cambium and secondary cortex together is called periderm.

The cork is impervious to water and air due to deposition of suberin in the cell walls.

Therefore at certain regions, the cork-cambium cuts of loosely arranged parenchymatous cells (known as complementary cells),

It soon rupture the epidermis, forming a lens-shaped openings called lenticels, through which gaseous exchange takes place.

 

In dicot root, the vascular cambium is entirely secondary in origin.

The cells located just below the phloem bundles and cells of pericycle above the protoxylem region, regain the capacity to divide and a wavy ring is formed which later becomes circular.

Secondary xylem is produced inside and secondary phloem outside like stem, and the diameter of root increases.

Secondary growth takes place in stem and roots of Gymnosperms also.

Gymnospermic wood is nonporous because it does not have vessels.

It is commercially called soft wood as it lacks xylem fibres.