The plants whose seeds have only one cotyledon are called dicots. The structure of dicot root varies greatly from that of the monocots. By understanding the structure of dicot root and monocot root, we can make comparisons between them and distinguish them by studying them under a microscope.
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Anatomy of Dicot root (gram)
Dicot roots of gram shows following distinct region in its Transverse section with following features:
- Vascular bundles
fig- T.S. of dicot root (gram shoeing its internal tissues organization
1. Epiblema or Epidermis – It is the outermost unilayered with several unicellular root hairs. It consists of thin walled, compactly arranged living parenchymatous cells. Usually epiblema is characterised by absence of stomata and cuticle. Sometimes the epiblema may be less cuticularised. It provides protection to the roots due to presence of unicellular root hairs it also helps in absorption of water and minerals from soil.
2. Cortex – It is thin walled, multilayered region made from circular or polygonal parenchymatous cells. they usually have intercellular spaces. The cortical cells have no chloroplast but may contain leucoplast for storage of starch grains. The cortex is responsible for transportation of water and salts from the root hairs to the center of the root.
3. Endodermis – It is the innermost layer of cortex and covers the stele. It consists of compactly arranged barrel shaped parenchyma without intercellular spaces. Most of the cells are characterised by the presence of special thickening of suberin and lignin on their radial and tangential walls called casparian strips. Some endodermal cell near protoxylem has no casparian strips and called passage cells or transfusion cells. These cells allow radial diffusion of water and minerals through the endodermis.
4. Pericycle – It is the outermost layer of stele and composed of uniseriate layer of parenchymatous cells without intercellular spaces. Some dicots and hydrophytes do not bear pericycle. Several lateral roots and lateral meristem arise from pericycle region (hence lateral roots are endogenous in origin). At the time of secondary growth, it produces secondary cambium or phellogens.
5. Vascular bundles – They are 2-8 in number, radial and arranged in ring. Xylem and phloem bundles are separated from each other by parenchymatous cells called conjuctive or complementary tissue.
- Xylem is exarch (i.e. protoxylem towards the periphery and metaxylem towards the centre) and consists of tracheids, vessels, xylem parenchyma and xylem fibres.
- The pholem forms oval masses beneath the pericycle, alternating with xylem bundles. Pholem consists of sieve tubes, companion cells and pholem parenchyma. Usually pholem fibres are absent or reduced.
6. Pith – it is feebly developed and centrally located. It consists of thin walled, polygonal parenchyma cells with intercellular spaces. In dicots roots, it may be reduced or absent. It helps in storage of food materials.
Distinguishing Features of Dicot Root
The typical dicot roots show following features.
- Epiblema is uniseriate, thin walled, colourless without intercellular spaces and produce unicellular root hairs, hence also called as piliferous layer or rhidodermis.
- Cortex is homogenous (without differentiation).
- Endodermis consists of barrel shaped compact parenchymatous cells. It contains both casparian stripes and passage cells.
- Pericycle uniseriate and become meristematic to give secondary roots and secondary tissues.
- Vascular bundles are radial; Xylem is exarch, number of xylem bundles varies from 2 to 4 rarely more (upto 6-8). Metaxylems are angular arranged in linear.
- Usually conjuctive tissues are well developed.
- Pith is very small or completed obliterated.