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  Structure of Dicotyledonous Root

Introduction to Primary Structure of Dicotyledonous Root - Bean Root

The transverse part of the dicot root (Bean) displays the following plan of arrangement of tissues from the periphery to the centre.

Rhizodermis or epiblema

The outermost layer of the root is termed as rhizodermis.  It is built up of a single layer of parenchyma cells that are arranged efficiently without intercellular spaces. It is devoid of cuticle and stomata.  Root hair is the entire time single celled. Root hair absorbs water and mineral salts from the soil. The major task of rhizodermis is protection.

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Diagram:  T.S. of Bean root

Cortex

Cortex contains only parenchyma cells. These cells are freely arranged along with intercellular spaces to create gaseous exchange easier.  These cells might store food reserves.   The cells are oval / rounded in shape. Occasionally they are polygonal because of mutual pressure.  Hence chloroplasts are not present in the cortical cells, starch grains are stored in them.  The cells as well possess leucoplasts.

The cortex's most inner layer is endodermis.  Endodermis is built up of single layer of barrel shaped parenchymatous cells.  Stele is entirely surrounded through the endodermis. The radial and the inside tangential walls of endodermal cells are thickened with suberin.   This thickening was firstly noticed by Casparay.  Thus these thickenings are termed as Casparian strips. But these casparian strips are not present in the endodermal cells that are positioned opposite to the protoxylem elements.  These thin- walled cells with no casparian strips are called passage cells by which water and mineral salts are conducted from the cortex to the xylem elements. Water cannot pass via other endodermal cells because of the existence of casparian thickenings.

Stele

All the tissues exist in endodermis comprise the stele. It involves vascular and pericycle system.

Pericycle

It is usually a single layer of parenchymatous cells found inner to the endodermis.  It is the most outer 'layer of the stele.  Lateral roots initiated from the pericycle.  So, in origin the lateral roots are endogenous.

Vascular system

Vascular tissues are in radial arrangement.  The tissue through which xylem and phloem are separated is termed as conjunctive tissue. In bean, the conjunctive tissue is formed of parenchymatous tissue. In exarch condition the Xylem is.  The no. of protoxylem points is four and thus the xylem is termed as tetrarch. Every phloem patch contains sieve tubes, companion cells and phloem parenchyma. Metaxylem vessels are usually in shape polygonal. But in monocot roots they are circular in shape.

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