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