THE ROOT

CHARACTERS

descending no green portion

develops from radicle

apex - sub-apical
protected by root cap

unicellular root hair

no nodes or internodes

lateral branches arise from Pericycle (endogenous origin)

negatively phototropic

positively

  • geotropic
  • hydrotropic

REGIONS OF A TYPICAL ROOT

ROOT CAP

  • parenchymatous multi cellular structure
  • secrete mucilage that lubricates the passage of root through soil
  • outer cells peeled off as they grow replaced by new cells formed from root meristem
  • replaced into Root Pockets in
    Lemna, Pistia
  • Pandanus aerial slit roots have multiple root caps

MERISTEMATIC REGIONS

  • 1mm long
  • actively dividing meristems
  • cells - thin, large nucleus, non vacuolated
  • absorption of mineral salts

ZONE OF ELONGATION

  • 5mm long
  • cells elongate rapidly and loose power of division
  • absorb water and mineral salts
  • vacuolated cells - cell sap
  • differentiated

ROOT HAIR

  • 1-6 cm
  • water absorption
  • cells differentiated to form xylem, phloem, pericycle, endodermis, cortex and epiblema
  • if cells mature, the root hairs shrivel and become non functional, ceasing to grow
  • new root hairs appear in zone of elongation

ZONE OF MATURE CELLS

  • no change in cells
  • outer layer, periderm covered with thick cuticle
  • no water absorbed
  • provides mechanical support

TYPES OF ROOTS

TAP ROOT - Dicots

primary root + branches

secondary --- tertiary ---- rootlets ---- root hairs

appear from radicle of embryo

Acropetal succession
old branches near base (thick) and new near apex(thin)

Perennial plants - deep feeders
rest - surface feeders

ADVENTITIOUS ROOTS - Monocots

roots developing from everywhere except radicle

SEMINAL ROOTS

replace the primary root if it is growing weakly or is short lived.

form the fibrous root system

MODIFICATIONS

surface feeders

uniform in thickness

TAP ROOTS

For food

  • primary tap may get swollen
  • hypocotyl stores food while secondary roots remain thin
  • stems: reduced to narrow green discs
  • leaves from such stem - Radical leaves

Pneumatophores - Mangrove

  • respiratory roots
  • plants growing in saline swamps
  • negatively geotropic
  • grow vertically up
  • lenticels / pneumathodes for exchange of gas

Sonneratia

Avicennia

Rhizopora mangle

Conical - Daucus Carota

broad base tapers towards apex

fleshy basal - hypocotyledon

secondary roots present

Fusiform - Raphanus Sativus

middle swollen tapers towards both ends

Secondary roots on apical part

spindle structure

root hair present on the bottom part

Napiform - Brassica rapa, Beta Vulgaris

spherical / globular

Primary root - thickest at base and tapers towards apex

upper part - hypocotyledon

root hair towards apex

ADVENTITIOUS ROOTS

Floating and Respiratory - Jussiaea

  • develop from stem nodes
  • white, spongy,abv water level
    -makes plant light

Food

Tuberous Roots - Ipomoea batatas

  • modified into tuberous or swollen structure
  • single no definite shape
  • adventitious buds

Fasciculated roots - Dahlia, Asparagus

  • several tubular roots in cluster

Mechanical Support

Stilt Roots
Zea mays, Saccharum Officiarum,
Pennisetum typhoides

  • from lower nodes of stem

Prop Roots - Ficus benghalenis

  • aerial roots
  • root caps present

Climbing Roots - Piper, Beetle, Ivy

  • arise from nodes
  • twine around support
  • produce viscous substance
  • adhesive discs

Photosynthetic / Assimilatory -
Tinospora, Trapa

  • aerial
  • phtosynthesis - have chlorophyll

Epiphytic - Orchids, Vanda, Dendrobium

  • aerial
  • velamen, spongy tissue
  • absorbs moisture from atmosphere

Haustorial - Cuscuta, Viscum

  • penetrate into host till xylem and phloem and absorb nourishment
  • partial parasite - absorb only water and minerals

Nodulated - Leguminous plants

  • Rhizobium inside cortical cells
  • pink due to leghaemaglobin