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 systemFibrous root system
Types of root systemsTypes 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 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 regionElongation region
Meristematic regionMeristematic region
Root capRoot cap
protodermprotodermground meristemground meristemprocambiumprocambium
Actively dividing cellsActively dividing cells
Produces mucigel forProduces mucigel forprotection, lubrication,protection, lubrication,water and nutrient absorptionwater and nutrient absorption
ROOT ROOT HAIRSHAIRSROOT ROOT HAIRSHAIRS
Increase the surface areaIncrease the surface area for the absorption of waterfor the absorption of water
Roots-ExternalRoot cap -indicates growth of new cells
Root hairs -absorb moisture (water) and minerals
Root development
Dicot Monocotthere 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 Monocot1. 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 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
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 plantsRoot images from a rice
plant
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 rootYoung dicot rootepidermisepidermis
cortexcortex
vascular cylindervascular 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 rootMonocot root
actinosteleactinostelepolyarchpolyarchexarexarchch exarexar
chch
metaxylem metaxylem vesselvessel
protoxylemprotoxylemvesselvessel
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 rootusually 3-5 groups of phloem and xylem arranged alternately
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