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Internal Structure of Root

Updated on October 5, 2015
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Anatomy deals with the study of gross internal details of plant organs like root, stem and leaf etc. the Dicot (primary), Monocot roots and stems in the transverse section show three main zones. They are epidermis, cortex and stele. The epidermis is the outermost layer which gives protection to the inner tissues. Cortex is present below the epidermis and it is composed of thin walled living parenchymatous tissue which helps in storage of food, assimilation etc. while the central conducting cylinder is knows as ‘stele’ and it is mainly composed of vascular tissues of different types of tissues and their functions. Let us discuss about the different zones in the internal structure of plant organs.

Internal structure of primary Dicot root:

In transverse section it shows three distinct zones.

1. Epidermis:

Is the outermost layer of layer of thin walled, rectangular living cells that are arranged closely and compactly arranged without intercellular spaces?

Some of the cell show tubular extensions called ‘root hairs’. Due to the presence of root hairs the epidermis is called as epiblema or hypodermis or piliferous layer. The cells that give rise to root hairs are called tri choblasts. The cuticle layer and stomata are absent. The root hairs grow in the soil and absorb water.

2. Cortex:

Is present between the epidermis and the stele. It is differentiated into

i. Exodermis:

Is the outer cortex composed of thick walled, suberised dead cells? When the epidermis is removed, it acts as the projective layer. It prevents the leakage of water through the cortex.

ii. General Cortex:

Is present beneath the exodermis and it is composed of several rows of thin walled living parenchymatous tissue. It store food materials and also helps in the lateral conduction of water into the stele.

iii. Endodermis:

Inner most layer of the cortex composed of a single row of barrel shaped cells arranged compactly without intercellular spaces.

These cells are characterized by the presence of lingo submersed bands on their radial and transverse walls casparian strips which prevent the movement of water. Endodermis acts as a barrier between the cortex and stele.

The thin walled endodermal cells situated opposite to the protoxylem elements are thin walled are called passage cells. They allow the entry of water and mineral salts from the cortex into the xylem.

3. Stele:

It is smaller in size than the cortex. It is consists of the following parts.

i. Pericycle:

It is usually uniseriate and composed of rectangular living cells which show active cell divisions. It gives rise to lateral roots and can differentiate into cambium which helps in the secondary growth of the root.

ii. Vascular bundles:

Vascular tissues are found alternatively on separate radii hence they are called separate or radial. Xylem is exarch showing protoxylem towards the pericycle and Meta xylem towards the medulla. Xylem is variable from monarch to octarch (one to eight) usually tetrarch.

Monarch-Trapa, diarch-Nicotiana, triarch-Pisum, tetrarch-Gossypium, pentarch-Ricinus, octarch-Castania.

Conjunctive tissue:

It’s the parenchymatous ground tissue extending between the xylem and phloem is called it helps in the storage of food materials. It produces secondary cambium during secondary growth.

iii. Medulla:

It is very small or may be completely absent.

The internal structure of Monocot root:

Is almost similar to the internal structure of dicot root with a slight variation in stellar structure. Usually in monocot root more than six xylem bundles are found. This is called ‘polyarch’ condition. Cortex and medulla are relatively bigger. Secondary growth is absent.

Internal structure of primary Dicot stem:

Show three main zones.

1) Epidermis:

Externally covered by a waxy layer called cuticle. It consists of minute pores called stomata and multi cellular uni or biseriate hairs called trachoma’s. The cuticle and trichomes check the evaporation of water and protect the stem from high temperatures.

2) Cortex:

It shows three sub zones.

a. Hypodermis:

The outer part of the cortex and is composed of collenchymatous cells. It gives tensile strength and helps in the production of food materials.

b. General cortex:

Found beneath the epidermis composed of thin walled spherical of oval parenchymatous cells. It is concerned with assimilation and storage of food materials.

c. Endodermis:

It is the innermost layer. Cells contain large number of starch grains. Hence it is also called starch sheath. Casparian strips also present.

3) Stele:

It is composed of four parts

a) Pericycle:

It is the outermost layer. It is made with thick walled, dead lignified cells and gives mechanical strength.

b) Vascular bundles:

Xylem and phloem are together found on the same radius and

cambium is present between them. So they are called Conjoint, collateral, open vascular bundles. Vascular bundles are wedge shaped and arranged in the form of a circle. This arrangement is called eustele. Eustele is considered as a primitive character. Xylem consists of vessels and xylem parenchuma. There may be few tracheids and xylem fibres. Xylem is endarch as protoxylem is present towards the centre. Phloem consists of sieve tubes and companion cells and phloem parenchyma.

c) Medulla:

It is the central part of the stele and very big in size. It is composed of thin walled parenchymeatous cells and helps in storage of food.

d) Medullary rays:

Are found in between the vascular bundles. They show living parenchymayous cells which help in lateral conduction food materials from cortex into the stele.

Internal structure of Monocot stem:

Epidermis:

Is the outermost layer composed of tubular or rectangular arranged compactly without intercellular spaces. Trichomes are absent.

Hypodermis:

Found beneath the epidermis. It is made up of dead, thick walled lignified sclerenchmatous cells. It gives mechanical strength.

Ground tissue:

It is the major part and is formed by an extensive soft, parenchymatous tissue. It is concerned with the storage of food materials. Endodermis is absent.

Vascular bundles:

Are numerous, oval in shape, found scattered irregularly in the ground tissue. This kind of arrangement is called atactostele. It is considered as an advanced character.. the inner vascular bundles are bigger and the outer bundles are smaller in size.

There is no cambium between the xylem and phloem. Hence the vascular bundles are conjoint, collateral and closed. Each vascular bundle is enclosed by a sheath of sclerenchmatous fibres. Hence it is called fibro-vascular bundle.

Xylem consist of trachieds, vessels, fibers and xylellm parenchyma. In the xylem, vessels are very few in number.

Xylem is arranged in the shape ‘Y’. out of four xylem vessels, two are meta xylem and two are protoxylem vessels.

One or two protoxylem vessels are crushed forming lysogenous cavity. It is called protoxylem lacuna. It stores water. Phloem has sieve tubes and companion cells but phloem parenchyma is absent. Medulla, medullary rays and pericycle are absent.

Internal structure of dorsiventral (Dicot) leaf:

It shows three main parts.

I. Epidermins:

On both sides is made up of a single row of barrel shaped cells arranged closely and compactly without the intercellular spaces. The outer surface of epidermis is covered by a waxy layer called cuticle. There are minute pores called stomata present on the epidermis. The number of stomata more in the lower epidermis than the upper epidermis. Each stoma is surrounded by two specialized kidney shaped cells called the guard cells. They regulate the opening and closing of stomata.

II. Mesophyll:

Is present between the upper and lower epidermis it is the assimilatory tissue and take part in the synthesis of carbohydrates. It is differentiated into palisade and spongy tissues. Palisade parenchyma: it is present beneath the upper epidermis composed of elongated compactly arranged columnar cells contains large number of choroplasts. It is mainly concerned with assimilation.

Spongy parenchyma:

It is present towards the lower epidermis. Cells are irregularly shaped, loosely arranged with large intercellular spaces. Cells have less number of chloroplasts. It facilities gaseous exchange and helps in synthesis of food materials.

III. Vascular bundles:

They are conjoint collateral and closed. Xylem is present towards the upper epidermis and phloem towards the lower epidermis.

Internal structure of isobilateral (monocot) leaf:

It shows three main parts called epidermis mesophyll and vascular bundle.

Epidermis:

Is present on both the upper side and lower side of the leaf. The number stomata is equal on the both the upper and lower epidermis. In grasses the upper epidermis shows specialized thin walled water storage cells bulliform cells. They help in rolling of the leaf. Mesophyll is undifferentiated. Mesophyll cells are chlorophyllous and the number chloroplasts is almost equal in all of cells. Hence leaves of monocots are equally green on both the surface. These leaves are called isobilateral leaves. It is the chief photosynthetic tissue of the leaf.

Vascular bundles:

Are conjoint, collateral and closed. They are surrounded by specialized mesophyll cells called bundle sheath. They show the lingo suberised bands suggesting the endodermal nature.

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