The internal structure of root is comparatively simple than that of the stem, is a simple organ and lacks any lateral appendages and nodes.
The primary structure the root in a transverse section shows certain distinct characters.
(i) The outermost layer of the root is known as epiblema or piliferous layer. It is not covered by cuticle.
(ii) Epidermal hairs or root hairs are unicellular
(iii) Hypodermis is absent.
(iv) Cortex is a broad zone.
(v) Endodermis is with characteristic casparian strips which occur outside the vascular bundles.
(vi) Vascular bundles are radial.
(vii) Xylem is exarch.
The detailed anatomy of dicot and monocot roots is described separately.
(A) Dicot root:
Dicot roots show the following distinct anatomical features.
(i) Number of vascular bundles varies from 2-6.
(ii) The pith is either reduced or absent.
(iii) Old roots exhibit secondary growth.
Internal structure of young root of gram (Cicer arietinum):
A thin transverse section of the root reveals the following internal structure under microscope.
1. Epiblema: This is the single outermost layer of thin walled cells.
Some of these cells give rise to unicellular root hairs from their outer surface.
2. Cortex: Inner to epiblema, there occurs multilayered thin walled parenchymatous cortex with conspicuous intercellular spaces.
3. Endodermis: The endodermis is the innermost layer of cortex. It surrounds the stele as a cylinder. It consists of closely packed barrel shaped cells. Radial walls and inner walls of this layer are often thickened. This thickening is known as casparian strips.
Inner to endodermis, there occurs pericycle. It consists of single layered, thin walled small parenchymatous cells.
5. Vascular Bundles:
Xylem and phloem occurring in separate patches arranged in a ring.
Vascular bundles are four in number arranged on alternate radii.
Xylem is exarch i.e. proto xylems towards periphery and Meta xylems towards centre.
The xylem consists of vessels.
The phloem consists of sieve tubes, combanion cells and phloem parenchyma. 6. Conjunctive Tissue:
The parenchymatous cells lying between xylems and phloem bundles are known as conjunctive tissue.
7. Pith – Pith is absent a very narrow zone of parenchyma. In old roots pith is not seen.
B. Monocot root:
The roots of monocotyledons show the following anatomical feature
(i) Number of vascular bundles is more than six.
(ii) The pith of monocot root is broader than that of dicot root.