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Root anatomy

The document describes the external and internal structures of roots. Externally, roots are cylindrical structures that lack nodes, leaves, and other aerial structures. Internally, roots have three longitudinal regions - a root cap region, a region of cell elongation, and a region of cell maturation. Roots also have two main types of root systems - a taproot system with a prominent primary root, and a fibrous (diffuse) system with many adventitious roots. The internal structure of young dicot and monocot roots is also compared, noting differences in vascular tissue arrangement and development.

Science◦
DR. DILIP V. HANDE PROFESSOR DEPT OF BOTANY SHRI SHIVAJI SCIENCE COLLEGE AMRAVATI SEM- III
EXTERNAL STRUCTURE OF ROOTS ▪ Roots  cylindrical structures  no nodes and internodes  do not generally bear buds, leaves, flowers and fruits  but the enlarged roots of sweet potato give rise to adventitious buds
EXTERNAL STRUCTURE OF ROOTS ▪ Classification of Roots based on Origin:  Primary root – first root formed from the embryonic root (radicle, growing tip of the hypocotyl)  Secondary root – roots arising from primary roots  Tertiary roots – arising from secondary roots  rootlets – branches of tertiary roots  Adventitious roots – arising from structures other than the hypocotyl, such as stems and leaves
EXTERNAL STRUCTURE OF ROOTS ▪ Kinds of Root Systems  Taproot System  has prominent primary root  Fibrous (diffuse) System  Primary root is lost and replaced by numerous adventitious roots arising from the lower portion of the stem  Slender in form and are more or less equally prominent
Tap root system Fibrous root system Types of root systems The fibrous root system of monocots is a mass of slender roots and lateral branches that hold the plant secure in the soil. Tap root is common in dicots; the first or primary root grows straight down and remains dominant root of a plant; often fleshy and adapted to store food (e.g., carrots, beets)
EXTERNAL STRUCTURE: LONGITUDINAL REGIONS ROOTS ▪ Root cap region  thimble-shaped mass of cells at the tip of the root  for protective covering for the growing point as it pushes its way through the soil ▪ Embryonic (Meristematic) Region  actively dividing region (apical mersitem) ▪ Region of Elongation or Cell Enlargement  Cells increase in size, particularly the length
EXTERNAL STRUCTURE: LONGITUDINAL REGIONS ROOTS ▪ Region of Maturation or Cell Differentiation  region where cells are differentiated (cells attain their final structural characteristic and perform their respective functions)  In Dicots: divided into 3 zones:  Root Hair Zone – numerous hairs; young root  Zone of Primary PermanentTissue (young)  derived from apical meristems; fully differentiated (primary tissue)  Zone of SecondaryTissues (older)  Derived form the lateral meristems or cambia  In Monocots: only 2 zones:  Root Hair Zone (young)  Zone of Primary PermanentTissue (young but old at the base)
Elongation region Meristematic region Root cap protoderm ground meristem procambium Actively dividing cells Produces mucigel for protection, lubrication, water and nutrient absorption
ROOT HAIRS Increase the surface area for the absorption of water
Roots-External  Root cap - indicates growth of new cells  Root hairs - absorb moisture (water) and minerals
Root development Dicot Monocot ▪there is pith in the very center composed of parenchyma. Instead there is xylem tightly packed with branching arms looking somewhat like a star.The phloem fills in the spaces in between the arms ▪ the phloem and xylem are in loose rings. With the phloem towards the outside and the xylem towards the inside. There is no pith in the center. - The main difference is in the vascular cylinder or stele.
Root development Dicot Monocot 1. usually 3-5 groups of phloem and xylem arranged alternately 2. xylem endarch (relating to a xylem whose early development is toward the center) 3. cambium present 1. there r many vascular bundles alternately arranged 2. xylem exarch (relating to a xylem whose early development is away from the center and toward the periphery) 3. no cambium
INTERNAL STRUCTURE: CROSS SECTION OF A YOUNG DICOT ROOTS ▪ Three Regions:  Epidermal or Dermal Region (outer region)  Single layer of living cells; may or may not have root hairs  Cortex (middle region)  Consists of 2 zones:  Outer zone consisting of several layers of parenchyma  Inner zone consisting of a single layer of thick-walled living cells (endodermis)  Passage cells – cells of the endodermis opposite the xylem rays
INTERNAL STRUCTURE: CROSS SECTION OF A YOUNG DICOT ROOTS ▪ Three Regions:  Stele orVascular Cylinder (inner region)  Pericycle (outermost part)  Single layer of thin-walled living cells (parenchyma)  Primary Xylem  tracheary elements organized in the form of a star  direction of development from outer (tip of the xylem ray) towards the inside – pattern called exarch  Primary Phloem  Patches of tissues located between the rays or radial arms of the primary xylem  Vascular Cambium  Layer of meristematic cells between primary xylem and phloem
INTERNAL STRUCTURE: CROSS SECTION OF A OLD DICOT ROOTS ▪ Two groups of tissue which enter into the tissue composition of old dicot root:  Secondary vascular tissues  consists of secondary xylem and phloem  Periderm  Phellogen (cork cambium)  Phellem (cork)  Phelloderm (secondary parenchyma)
INTERNAL STRUCTURE: CROSS SECTION OF AN OLD DICOT ROOTS ▪ What happens? --- once differentiated:  Cambium gives rise to the secondary phloem outwardly and secondary xylem inwardly  Secondary xylem would eventually occupy the places formerly occupied by primary phloem  Secondary xylem → vascular cambium → secondary phloem  Pericycle becomes meristematic and gives rise to cork cambium (phellogen)  Cork cambium produces phelloderm inwardly and phellem or cork outwardly (serves as the outermost tissue)
INTERNAL STRUCTURE: CROSS SECTION OF A MONOCOT ROOTS ▪ Three Regions:  Epidermal or Dermal Region (outer region)  Cortex (middle region)  Stele (inner region) – consists of  Pericycle  Xylem – star-shaped like of a young dicot, except for:  star-shaped xylem has more radial arms (polyarch)  center of star-shaped xylem may be occupied by sclerenchyma fibers and parenchyma; tracheary elements are found only within the radial arms  Phloem – occupies the spaces between the radial arms of xylem  No vascular cambium
Fig. 35-7, p. 753
Young dicot root epidermis cortex vascular cylinder endarch relating to a xylem whose early development is toward the center) there is pith in the very center composed of parenchyma. Instead there is xylem tightly packed with branching arms looking somewhat like a star. The phloem fills in the spaces in between the arms
Monocot root actinostele polyarch exarch exarch metaxylem vessel protoxylem vessel exarch - xylem whose early development is away from the center and toward the periphery
Monocot root -there are many vascular bundles alternately arranged - the phloem and xylem are in loose rings. With the phloem towards the outside and the xylem towards the inside. There is no pith in the center.
Dicot root usually 3-5 groups of phloem and xylem arranged alternately