Describe the internal structure of a Dicot Root .

Explanation

The transverse section of primary dicto root can be divided into 3 zones . They are :Epidermis , Cortex and Stele .
Epidermis : Epidermis is the outermost layer made up of thin walled , non - cutinised , rectangular living cells . The epidermal cells are arranged compactly without intercellular spaces . The cuticle and stomata are absent . Some epidermal cells produce tubular extensions called root hairs . Due to presence of root hairs the root epidermis is called is as epiblema or rhizodermis or piliferous layer . The cells that give rise to root hairs are comparatively smaller than the other cells and are called trochoblasts . Root hairs help in the absorption of capillary water . The epidermis gives protection to the inner tissues and plays major role in the absorption .
(2) Cotex : The tissue extended between epidermis and stele is called .Generally in roots the cortex is bigger than the stele . Cortex can be differentiated into three parts : (i) Exodermis ,(ii) Genaral cortex and (iii) Endodermis .
(i) Exodermis : It is the outermost layer of cortex and composed of two to three rows of thick walled subrised cells . When the epidermal layer is removed , the exodermis acts as the protective layer . It also prevents the exit of water from the cortex . It can be observed in mature part of the root . (ii) General cortex : It is present beneath the exodermis and is composed of several rows of thin walled , living parenchyma cells . The cells are round or oval in shape and loosely arranged showing intercellular spaces. They contain leucoplasts which store the food materials . the general cortex helps in the lateral conduction of water from the epidermis to the xylem vessels present in the state .
(iii) Endodermis : It is the innermost layer of cortex and is made up of a single row of barrel shaped cells . The cells are compactly arranged without having any intercellular spaces . The radial and transverse walls of the endodermal cells show casparian strips that are formed by the deposition of lignin and subein , which prevent the movement of water . Therefore the endodermis acts as a barrier between the cortex and the stele .
In endodermis , some cells situated opposite to the protoxylem elements are thin walled without casparian bands . These cells are called passage cells . they help in the translocation of water and mineral salts from the cortex into the stele .
(3) Stele : the central conducting cylinder is known as .stele . . It is smaller than the cortex . the stele is comprised of three parts , viz., pericycle , Vascular bundles and Medulla .
(i) Pericycle : The central conducting cylinder is known as stele . It is smaller than the cortex . The stele is comparised of three parts , vix., pericycle , Vascular bundles and Medulla .
(i) Pericycle : the layer of cells surrounding the stele is known as .pericycle . .It is usually uniseriate and composed of thin walled , rectangular , living cells which show active cell division . The pericycle gives rise to lateral roots . Some cells of the pericycle can dedifferentiate into secondary cambium which results in the secondary growth of the root .
(ii) Vascular bundles : Standards of primary xylem and phloem are found alternatively on separate radii. These are called separate radii vascular bundles . Xylem is exarch showing protoxylem elements towards the pericycle and metaxylem elements towards the medulla . The number of vascular bundles is identified in relation to the number of xylem groups , Usually in dicot roots , four sylem bundles alternating with four phloem bundles are found . This is called .tetrach condition. .
There is no cambium between the vascular tissues . The ground tissue that extends between the xylem and phloem strands is called conjunctive tissues . The ground tissue that extends between the xylem and phloem strands is called conjunctive tissue , It is usually parenchymatous . it helps in the storage of food materials . It produces secondary cambium during secondary growth .
Medulle or Pith : In roots , the development of xylem is centripetal and produces mataxylem towards the inner side . Sometimes , as in dicot root these metaxylem elements come closer from all sides and replace the -medulla .Hence in dicot roots the medulla is very small or may be completely absent . When present , it is parenchymatous and helps in the storage of food and water .