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
Types of root systems
Tap root system
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)
Fibrous root system
The fibrous root system of monocots is a
mass of slender roots and lateral branches
that hold the plant secure in the soil.
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 Permanent Tissue (young)
 derived from apical meristems; fully differentiated (primary tissue)
 Zone of Secondary Tissues (older)
 Derived form the lateral meristems or cambia
 In Monocots: only 2 zones:
 Root Hair Zone (young)
 Zone of Primary Permanent Tissue (young but old at the base)
Elongation region
protoderm
ground meristem
procambium
Meristematic region
Actively dividing cells
Root cap
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
- The main difference is in the vascular cylinder or stele.
Dicot
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
 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.
Root development
Dicot
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
Monocot
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 or Vascular 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
FUNCTIONS OF ROOTS
 Support
 presence of brace roots, prop roots, clinging roots, and buttresses
(extensions from the lower part of stem)
 Food storage
 enlarged, fleshy or succulent roots
 Photosynthesis
 green aerial roots
 Reproduction
 Development of adventitious bulbs
 Protection
 Presence of spines
 Aeration
 Development of pneumatophores (roots with spongy tissues protruding
above the ground
Functions of Roots
Root images from a rice
plant
 Absorption-take water
and nutrients from
the soil and conduct
them to the stem
 Anchor the plant and
hold it upright
 Store food for plant
use
 Asexual reproduction
in some plants
List at least 5 practical
human uses for roots
 Food storage
 sugar, beets, turnip, rutabagas, parsnip, radish, carrot
 Spices
 Sassafras, sarsaparilla, licorice,
 Dyes
 reds, browns, coffee bean
 Drugs
 gentian, reserpine (tranquilizer)
 Insecticide
 rotenone
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
metaxylem
vessel
exarch
protoxylem
actinostele polyarch
exarch
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