The following points highlight the six main parts of primary dicot root in plants. The parts are: 1. Epiblema 2. Cortex 3. Endodermis 4. Pericycle 5. Vascular Strand 6. Pith.
Dicot Root: Part # 1. Epiblema or Piliferous Layer (Rhizodermis):
It is the outermost layer of the root. It is made of compactly arranged thin-walled flattened and slightly elongated parenchymatous cells. Epiblema of root differs from the epidermis of stem in being devoid of distinct cuticle and stomata. Some cells of the epiblema give rise to thin-walled tubular outgrowths called root hairs. They are called trichoblasts. Trichoblasts are generally smaller than other epiblema cells.
The root hairs lie in between the soil particles and are in contact with the soil water. Root hairs possess a gummy pectic layer on the outside for cementing with soil particles and retaining water on the surface. Due to the presence of root hairs, the epiblema is also called piliferous layer (L. pilus— hair, ferre— to carry).
The root hairs and thin- walled epiblema cells absorb water and minerals salts from the soil. Root hairs commonly do not live for more than one week. With their death the epiblema cells become suberized and cutinised.
Dicot Root: Part # 2. Cortex.
It lies below the epiblema. The cortex is made up of many layers of thin walled parenchyma cells. The parenchyma cells may be rounded (e.g., Cicer) or angular (e.g., Sunflower). They enclose intercellular spaces for diffusion of gases. The cells of the cortex store food. They also conduct water from the epiblema to the inner tissues.
Dicot Root: Part # 3. Endodermis:
Endodermis is usually considered to be the innermost layer of the cortex. It is made up of a single layer of barrel-shaped cells which do not enclose intercellular spaces. The cells are rich in starch grains.
The young endodermal cells possess a band of thickening which runs along their radial and tangential walls. This band of thickening is called casparian strip (after Caspary, 1865). It is made up of both suberin and lignin (Esau, 1965).
In a transverse section, the casparian strip appears in the form of small lenticular swellings on the radial walls only. Casparian strips prevent plasmolysis of endodermal cells. Due to the presence of casparian strips, the endodermal cells do not allow wall to wall movement of substances between cortex and pericycle.
Substances must enter the cytoplasm of endodermal cells. As a result, endodermis functions as a biological check post. All tissues on the inner side of endodermis constitute stele. It consists of pericycle, vascular bundles and pith.
Dicot Root: Part # 4. Pericycle:
Endodermis is followed by one (e.g., Sunflower) or more (e.g., Mulberry) layers of pericycle. Pericycle is believed to represent the outer boundary of vascular strand. The cells of pericycle are thin-walled and parenchymatous in the young root.
Pericycle is a very important layer. A part of the vascular cambium is formed by the pericycle. The cork cambium also develops from it. All lateral roots originate from the pericycle. Pericycle is absent in the roots of some aquatic plants and parasites.
Dicot Root: Part # 5. Vascular Strand or Cylinder:
Inner to the pericycle are found a few (2-6) alternately arranged bundles of xylem and phloem. They are equal in number and lie on different radii. Such vascular bundles are called radial bundles. The various xylem bundles put together give a stellate or star-shaped appearance. The number of rays is equivalent to the number of xylem bundles (and phloem bundles).
According to the number of rays, the root may be diarch (with 2 xylem bundles, e.g., Tomato), triarch (Pea), tetrarch (Buttercup, Gram, Sunflower, Castor), pentarch (with 5 xylem bundles) or hexarch (with 6 xylem bundles). In some cases different divisions of roots possess different number of strands, e.g., two in lateral roots and four in main roots of Garden Nasturtium. Such roots are called heteroarch.
Protoxylem or the first formed xylem lies in contact with pericycle and at the tip of the rays while metaxylem or later formed xylem is present towards the centre of the root. Such a xylem is called exarch (L. ex— outside, Gk. arche— beginning).
The metaxylem elements of different xylem bundles may lie separate from one another so that a pith is present in the centre of the root (e.g., Gram, Bean). However, usually the xylem bundles extend along the radii so that metaxylem elements of different bundles meet in the centre to form a solid star-shaped structure. In such a case the pith is absent.
Xylem is made up of vessels and a few tracheids. Vessels and tracheids are polygonal in outline. Protoxylem elements are fewer, smaller and narrower. The metaxylem elements are larger and wider. They have pitted thickenings while protoxylem possesses spiral, annular, reticulate or scalariform thickenings.
Xylem performs two important functions:
(i) Mechanical strength,
(ii) Conduction of water and mineral salts to the shoot.
In between the two adjacent xylem bundles is found a phloem bundle. It is oval in outline. Phloem and xylem bundles are separated from each other by one or more layers of small thin walled cells called conjunctive parenchyma or tissue. Later on the conjunctive tissue becomes meristematic to form vascular cambium.
Phloem consists of sieve tubes, companion cells and phloem parenchyma. It conducts organic food from the shoot to the root and its branches. Fibres may occur outside the phloem in some roots (e.g., Gram). Radial arrangement of vascular bundles is a mechanism to keep the xylem bundles in direct contact with the outer tissues of the root which conduct water absorbed by the root hairs to the inside.
Dicot Root: Part # 6. Pith:
It is often absent. When present, the pith is quite small. The latter is made of parenchyma cells. Intercellular spaces are absent. The cells store food as well as waste materials.