Plant Anatomy – Roots (Primary Structure & Function)

Week-2 Focus – Roots

• Builds on Week-1 topics (embryogenesis → meristems → tissues).
• Goal: survey organ-level anatomy (roots, stems, leaves, secondary growth).
• Current lecture = in-depth study of roots (mostly primary, i.e., before secondary thickening).

Core Functions of Roots

• Anchor plant to substrate.
• Absorb water + mineral nutrients.
• Act as carbohydrate storage organs (reserve for next season).
• Serve as conduits transporting xylem sap upward and laterally.
• Often constitute ≥50 % of total plant biomass; hidden but extensive.
• Surface area can exceed shoot area and may extend \approx 2\times crown radius horizontally.

Growth Dynamics & Ecological Significance

• Fine roots are highly dynamic: continual turnover (death ↔ regeneration) while exploring new soil patches.
• Rooting depth = key ecological trait: determines soil volume & water table access; crucial in arid ecosystems.
• Case study (Texas caves, Will Pockman lab):
  • Installed sap-flow sensors on deep roots visible in cave walls.
  • Revealed bidirectional water movement:
  • Hydraulic lift: deep roots move water upward to dry upper layers via lateral roots.
  • Hydraulic redistribution (inverse): after rainfall, shallow roots channel water rapidly downward via root system.
  • Alters soil moisture profiles & interspecific competition.

Symbiotic/ Microbial Associations

• Mycorrhizas (most species): fungal hyphae extend absorptive reach; pivotal for P uptake.
• Nitrogen-fixing nodules:
  • Legumes + Rhizobium spp.
  • Non-legume examples (NZ):
  • \textit{Coriaria} (Tutu) + Frankia
  • \textit{Alnus} (alder, riverbanks) + Frankia
• Rule rather than exception: roots are a composite plant–microbe organ.

Macroscopic Layout of a Primary Root

• External landmarks (distal → proximal):
  1. Root cap
  2. Zone of root hairs
  3. Region where lateral roots emerge
• Internal developmental zones:
  1. Root cap (protection; lubrication)
  2. Meristematic zone (apical meristem + primary meristems)
  3. Cell-elongation zone
  4. Maturation zone (differentiation of tissues, root hairs start/finish)

Root Apical Meristem Types

• Open organization: common group of initials → indeterminate lineage boundaries (cells in a “ball”).
• Closed organization: discrete tiers of initials, each giving rise to a specific tissue layer (easily traceable lineages).

Root Cap Details

• Secretes mucilage ⇒ hydrated gel that lubricates soil penetration.
• Border cells slough off, further reducing friction and influencing rhizosphere microbes.
• Statolith-containing cells perceive gravity → maintain positive geotropism.

Dermal Tissue System

Epidermis

• Thin/ minimal cuticle (balance between protection & absorption).
• Root hairs:
  • Single-cell tubular outgrowths.
  • Enormously increase surface area; extremely short-lived.
• Rhizosphere: soil sheath influenced by exudates (mucilage, organic acids) that modulate pH, metal availability, microbiota.

Ground Tissue System – Cortex

• Usually parenchyma rich in starch; may be lignified in monocots.
• Large volume proportion in primary root; lost when secondary growth expands inward.
• Contains intercellular air spaces & abundant plasmodesmata ⇒ symplastic continuity.

Endodermis

• Innermost cortical layer; single cell thick.
• Characterized by Casparian strip (radial wall band of suberin/ lignin) which blocks apoplastic flow.
• Forces solutes to cross plasma membrane \Rightarrow living control of uptake/ exclusion.
• May later develop secondary wall thickenings (U- or O-shape) on inner face.

Exodermis (some species)

• Suberized layer just beneath epidermis; adds an outer barrier regulating fluxes.

Vascular Tissue System – The Stele (Vascular Cylinder)

• Derived from procambium; components:
  • Xylem, phloem (primary)
  • Pericycle (parenchyma cambial layer between vascular tissues & endodermis)
  • Optional pith (parenchyma) in siphonosteles.

Stele Types in Roots

1. Protostele (typical eudicots): solid core of xylem; star-shaped in cross-section.
2. Siphonostele (many monocots): ring of vascular tissue enclosing central pith.

Eudicot Example – Buttercup (\textit{Ranunculus})

• 4–5-armed star (tetra-/penta-arch):
  • Protoxylem poles at star tips.
  • Metaxylem maturing centripetally (center).
  • Primary phloem strands alternate with xylem arms.
• Endodermis encircles stele; pericycle sandwiched between endodermis & phloem/xylem.

Monocot Example – Maize (\textit{Zea mays})

• Polyarch siphonostele (10–20 poles):
  • Very large metaxylem vessels (eye-catching).
  • Phloem patches outward of each protoxylem.
  • Central pith of parenchyma.

Xylem Maturation Patterns

• Exarch (roots): protoxylem outside → maturation proceeds inward (metaxylem central).
  \text{root pattern} = \text{exarch}
• Endarch (stems): protoxylem nearest stem center; metaxylem toward periphery.
  \text{stem pattern} = \text{endarch}
• Diagnostic for distinguishing fossil/ extant roots vs stems; informs plant phylogeny (e.g., lycophyte fossils).

Pericycle – The Hidden Cambium

• Parenchyma layer just inside endodermis.
• Retains meristematic competence; functions:
  1. Origination of lateral roots (primary meristem replicates internally before emergence).
  2. Contributes to vascular cambium during secondary growth.
  3. Later forms part of cork cambium (phellogen) in mature, woody roots.

Lateral Root Formation Sequence

1. Cell division re-initiates in pericycle opposite protoxylem poles (often 2–3 adjacent cells).
2. Lateral root primordium forms, organized like miniature root apex.
3. Primordium pushes outward, penetrating endodermis, cortex, epidermis.
4. Emerges externally without disrupting primary root tip (no buds on elongating zone).

Key Take-Home Messages

• Roots = critical yet hidden organs: anchorage, absorption, storage, ecological water redistribution.
• Primary root has 4 distinct developmental zones; know cell activities in each.
• Three tissue systems (dermal, ground, vascular) appear in predictable layers; understand epidermis → cortex (with endodermis) → stele sequence.
• Stele architecture (protostele vs siphonostele) & xylem maturation (exarch vs endarch) are taxonomically and functionally significant.
• Pericycle is essential: initiates lateral roots and later contributes to secondary growth.