T.S. of dicot root (sunflower, Bean and pea) shows following internal structures:

Epiblema:

• It is also termed as rhizoderm or piliferous layer.
• It is outermost single layer of root which is composed of thin-walled, closely packed parenchymatous cells without intercellular spaces.
• The cuticle and stomata are absent.
• Most of epidermal cells extend out in form of tubular unicellular root hairs.
• Due to the presence of root hairs in epiblema, it is named as piliferous layer.
• This layer functions for the uptake of water and mineral salts from the soil and thus has no cuticle.
• Root hairs provide maximum surface area for absorption.

Cortex:

• It is located below the epiblema.
• It consists of many layers of thin-walled rounded or polygonal parenchymatous cells with sufficiently developed intercellular spaces between them.
• Cells of cortex consists of leucoplasts and store starch grains.
• Sometimes, outer layer of cortex becomes cutinized and forms exodermis of root.
• Cortex cells store food and conduct water from epiblema to the inner tissues.

Endodermis:

• It is the innermost layer, made up of single layer of barrel shaped compact parenchymatous cells without intercellular spaces.
• The radial walls of this layer are often thickened and sometimes this thickening extends to the inner walls also.
• Deposition of suberin and lignin causes the thickening.
• Due to deposition, strip or bands like structures are formed which are known as casparian strips or casparian bands.
• Cells of the endodermis that are located opposite the proto-xylem elements are thin-walled and termed as passage cells as they facilitate the passage of water from roots to the xylem.
• Endodermis acts as a watertight jacket around the stele.

Pericycle:

• It is located internal to the endodermis and made up of single layer of thin walled parenchymatous cells containing abundant protoplasm.
• It is very important layer as part of vascular cambium is formed from it.
• Lateral roots in dicot arise in this tissue and cork cambium also develops from it.

Conjunctive bundles:

• In between xylem and phloem bundles, there is presence of one or many layers of thin walled elongated parenchymatous cells without intercellular spaces constitutes the conjunctive tissue.
• It functions for storage of foods.

Vascular bundles:

• These are arranged in a ring but xylem and phloem form an equal number of separate bundles placed on different radii.
• As xylem and phloem are alternately arranged, the vascular bundles are termed as radial bundles.
• The number of xylem or phloem bundles varies from two to six, very rarely more.
• Xylem:
  • It appears in conical in shape.
  • The cells in T.S. appear polygon, and are thick walled.
  • The protoxylem lies towards the periphery, so the xylem is called exarch.
  • The protoxylem vessels bear annular and spiral thickenings while metaxylem vessels have reticulate and pitted thickenings.
  • In mature and much developed root, the metaxylem vessels meet in centre, and pith gets obliterated.
  • Xylem parenchyma and fibers are absent.
  • A few tracheids are available around the vessels.
  • Xylem is responsible for:
  • Conduction of water and mineral salts
  • Provides mechanical strength
• Phloem:
  • It lies alternate to xylem patches.
  • The patches are smaller and consist of sieve tubes, companion cells and phloem parenchyma.
  • The phloem fibers are absent.
  • The outerpart of this tissue next to pericycle is the protophloem and inner is metaphloem, but both are not easily distinguishable.
  • In the hard root, a few sclerenchyma cells occur against the patch of every phloem.

Pith:

• This occupies only a small area in the center and consists of few compactly arranged, thin-walled parenchymatous cells without any intracellular space.
• Sometimes the pith is nearly obliterated owing to the wood vessels meeting in the center.

Internal structure of dicot root

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