Primary Structure of Roots

Dicot Root

A thin transverse section of dicot root shows the following structures:

Epiblema: Single, outermost layer of thin-walled cells. Some cells are prolonged to form unicellular root hairs. It protects and absorbs water.

Cortex: Large zone, many layered, cells thin-walled parenchymatous with intercellular spaces, stores food and water.

Endodermis: Innermost layer of cortex, cells barrel-shaped, closely packed, that show band like thickenings on their radial walls called casparian strips. Some cells (opposite the protoxylem) which lack these strips are called passage cells. They help in the movement of water and dissolved salts from cortex directly into xylem.

Stele: All tissues inner to endodermis comprise stele.

Pericycle: Inner to endodermis lies a single layer of pericycle. It is the seat of origin of lateral roots and vascular cambium and cork cambium during secondary growth.

Vascular bundle: It consists of xylem and phloem patches lying on alternate radii i.e., it is radial. Xylem is exarch where protoxylem (first formed, having narrow vessels and tracheids) lies towards the periphery and metaxylem (differentiates later, has wider vessels and tracheids) lies towards the center. Depending upon the number of xylem patches a root may be diarch (di-2 patches) to hexarch (hexa-6 patches).

Pith: Sometimes the metaxylem of all xylem patches meet in the centre, and in that case pith is absent or is small and parenchymatous.

Conjunctive parenchyma: Parenchyma which separates xylem and phloem lying on different radii.

Monocot Root

A thin transverse section of monocot root shows the following structures:

Epiblema: Outermost, single layer of thin-walled, closely packed cells. Some cells are prolonged into unicellular root hairs.

Cortex: Large zone, multilayered, composed of parenchymatous cells with intercellular spaces, stores water and food material.

Endodermis: Innermost layer of cortex with characteristic casparian strips Plants and animals and passage cells.

Stele: All the tissues inner to endodermis constitute stele 

Pericycle: Single layered, having polygonal thin walled cells. The lateral roots originate from this layer.

Vascular bundle: It consists of many patches of xylem and phloem arranged radially. The xylem is exarch and polyarch (poly-many).

Pith: Is situated in the center, large, well developed, parenchymatous or sclerenchyamatous, stores food.

Conjunctive Parenchyma: Is located in between the strands of xylem and phloem.

Differences Between Dicot and Monocot Root

Character	Dicot root	Monocot root
Number of vascular bundles	2-6 (di-hexarch)	Many (polyarch)
Cambium	Present	Absent
Secondary growth	Present	Absent
Pith	Very small or absent	Large