The dicot root is the root system found in dicotyledonous plants, whose seeds contain two cotyledons. It typically develops into a tap root system, where the primary root persists and produces lateral branches throughout the plant's life. Dicot roots are responsible for water and mineral absorption, anchorage, storage of food, and conduction of substances. They are characterised by a limited number of vascular bundles and the presence of secondary growth, which increases the thickness of the root with age.
Structure of Dicot Root
The internal structure of a dicot root can be studied by examining its transverse section. The tissues are arranged in concentric layers, progressing from the outermost region toward the centre. These layers include the epiblema, cortex, endodermis, pericycle, vascular tissues, and pith.
1. Epiblema or Piliferous Layer
- The epiblema forms the outermost layer of the dicot root. It consists of a single layer of thin-walled, living, parenchymatous cells that are compactly arranged without intercellular spaces.
- The epiblema bears numerous unicellular root hairs that increase the absorptive surface area of the root. These root hairs absorb water and dissolved mineral salts from the soil solution and transport them into the inner tissues of the root.
- The epiblema lacks a cuticle because a waterproof covering would interfere with water absorption. Stomata are also absent because roots are not directly involved in gaseous exchange like leaves.
2. Cortex
- Beneath the epidermis lies the cortex, which is composed of several layers of thin-walled parenchymatous cells. The cortical cells possess conspicuous intercellular spaces that facilitate the movement of water and dissolved substances.
- The cortex serves as a storage region for food materials and water. It also provides mechanical support and acts as a pathway through which absorbed water and minerals move toward the vascular tissues.
3. Endodermis
- The innermost layer of the cortex is known as the endodermis. It consists of a single layer of barrel-shaped cells arranged compactly without intercellular spaces.
- The radial and transverse walls of the endodermal cells possess Casparian strips made of suberin and lignin. These strips regulate the movement of water and minerals into the vascular cylinder and prevent their uncontrolled passage through the cell walls.
- Certain cells of the endodermis located opposite the protoxylem remain thin-walled and are known as passage cells. These cells facilitate the movement of water and mineral ions into the vascular tissues.
4. Pericycle
- The pericycle is located immediately below the endodermis and forms the outermost layer of the stele. It consists of a single layer of thin-walled parenchymatous cells.
- The pericycle plays a significant role in the formation of lateral roots. In dicot roots, it also contributes to secondary growth by giving rise to parts of the vascular cambium and cork cambium.
5. Vascular Tissues
- The vascular tissues occupy the central region of the root and consist of xylem and phloem arranged alternately on different radii. This arrangement is known as a radial vascular bundle.
- The xylem conducts water and mineral salts from the roots to the aerial parts of the plant, whereas the phloem transports food materials synthesised during photosynthesis.
- The xylem exhibits an exarch condition, in which the protoxylem lies toward the periphery, and the metaxylem occupies a more central position. Depending on the number of xylem groups present, dicot roots may be diarch, triarch, tetrarch, pentarch, or hexarch. The tetrarch condition, where four xylem bundles are present, is particularly common.
- The xylem tissue is generally arranged in a star-shaped pattern at the centre of the root, with phloem patches situated between the arms of the xylem.
6. Pith
- The pith occupies the central region of the root. In dicot roots, the pith is usually very small, poorly developed, or completely absent.
- When present, the pith consists of a few thin-walled parenchymatous cells.
- The reduced nature of the pith is one of the distinguishing features that separates dicot roots from monocot roots, which possess a large and well-developed pith.
Characteristics of Dicot Root
- The root system is a tap root system.
- The primary root persists throughout the life of the plant.
- The epiblema bears unicellular root hairs.
- Cuticle and stomata are absent.
- The cortex is composed of parenchymatous cells with intercellular spaces.
- The endodermis contains Casparian strips and passage cells.
- The pericycle gives rise to lateral roots and contributes to secondary growth.
- Vascular bundles are radial in arrangement.
- Xylem is exarch in nature.
- The number of xylem bundles ranges from two to six.
- The pith is small, inconspicuous, or absent.
- Secondary growth is generally present.
Functions of Dicot Root
- The root hairs absorb water and dissolved mineral nutrients from the soil and transport them to other parts of the plant.
- The tap root system firmly anchors the plant in the soil and provides stability against environmental forces.
- The vascular tissues conduct water, minerals, and food substances between the roots and aerial parts of the plant.
- In many dicot plants, roots become modified for the storage of food materials that can be utilised during periods of growth and reproduction.
- Secondary growth increases the thickness of the root and enhances its conducting and supporting capabilities.