Plant Secondary Growth – Vocabulary Review

Lecture Context

• Third lecture (out of four) on the internal structure of plants.
  • First two lectures: roots, then stems.
  • Current focus: secondary growth (occurs in both roots and stems).
• Reference specimen shown: cross-section of a 4-year-old tulip poplar stem displaying 4 annual rings.
  • Dark reddish tissue = secondary xylem.
  • Inner green-stained band = phloem.
  • Outer layers collectively called bark (includes phloem + periderm).
• Textbook correlation: Chapter 26 (provides additional detail).

Primary vs Secondary Growth

• Primary growth
  • Increases length via root & shoot apical meristems.
  • Generates three primary meristems: dermal, ground, vascular.
• Secondary growth
  • Increases girth (thickening) after primary tissues form.
  • Plants with any woody, hard, or wiry stems possess secondary growth—even small shrubs (e.g.
    Coprosma), tomatoes, dwarf beans.
  • Plants entirely lacking secondary growth = herbs/herbaceous.
• Evolutionary context
  • Lignophytes: ancestral seed plants possessing wood.
  • Angiosperms secondarily lost woodiness in many lineages to become herbaceous.

Vascular Cambium: Structure & Function

• Definition: Secondary meristem that forms after primary meristems complete elongation.
• Cell types
  • Fusiform initials (elongated): produce axial tissues—tracheids & vessel elements (xylem) or sieve elements & fibers (phloem).
  • Ray initials (round/patch-like): produce radial parenchyma rays crossing xylem ↔ phloem.
• Division orientations
  • Periclinal (parallel to surface): dominates; adds secondary xylem inward & secondary phloem outward.
  • Anticlinal (perpendicular to surface): occasional; enlarges cambial circumference as stem/root diameter increases.
• Three-dimensional wood architecture arises from simultaneous axial (fusiform) and radial (ray) production.

Secondary Growth in Roots (Protostele Pattern)

• Starting point: primary root with solid vascular cylinder (protostele).
  • Primary xylem forms a star (commonly 3 or 4 arms).
  • Primary phloem located between xylem arms.
  • Procambium around xylem retains meristematic capacity.
• Cambium initiation
  • Begins between primary xylem & phloem (discontinuous patches).
  • Patches unite into a complete vascular cambium ring.
• Year-by-year progression
  • Inward production: continuous ring of secondary xylem; original star becomes a small central relic.
  • Outward production: secondary phloem; primary phloem pushed outward, often crushed.
  • Cortex cannot stretch indefinitely ⇒ ruptures & sloughs off.
  • Cork cambium (outside phloem) forms new protective periderm layers.
• Microscopic view: shows central primary xylem remnant, surrounding secondary xylem, narrow band of secondary phloem, outer pericycle/periderm.

Secondary Growth in Stems (Eustele Pattern)

• Primary stem begins as eustele = discrete vascular bundles (xylem inside, phloem outside).
• Cambium formation
  • Fascicular cambium: meristematic cells that remain inside each primary bundle.
  • Interfascicular cambium: new meristematic bands arising between bundles.
  • Together form a continuous cambial cylinder.
• Activity mirrors roots
  • Secondary xylem inward; secondary phloem outward.
  • Primary phloem fibers (hard caps) mark original bundle positions; remain visible as radial points.
• Aging sequence
  • 1-year stem: thin rings; cortex still intact; emerging cork cambium.
  • 3-year stem: visible 3 annual rings (earlywood vs latewood); well-developed periderm, multiple cork cambium generations possible.

Cork Cambium & Periderm

• Cork cambium (phellogen): secondary meristem replacing ruptured epidermis/cortex.
  • Produces cells outward → cork (phellem).
  • Produces cells inward → phelloderm.
  • \text{phellem} + \text{phellogen} + \text{phelloderm} = \text{periderm}.
• Multiple successive cork cambia ⇒ thick, layered bark (e.g.
  conifers with plate-like bark).
• Microscopy clues: thin-walled, small meristematic cells; frequent periclinal divisions.

Annual Rings & Wood Anatomy

• Earlywood: formed at season start; larger vessel/tracheid diameters for maximum water transport.
• Latewood: formed as growth slows; smaller cells; denser wood.
• Distinct boundary between one year’s latewood and the next year’s earlywood ⇒ visible ring.
• Sectional terminology
  • Transverse: cross-section perpendicular to stem axis (shows rings).
  • Radial: cut along radius (center → bark).
  • Tangential: cut parallel to growth rings, tangent to circumference.

Bark: Practical Definition & Implications

• Bark = all tissues external to the vascular cambium (secondary phloem + successive periderms).
• Ring-barking (girdling) removes this entire zone.
  • If phloem continuity not restored, photosynthate can’t move to roots ⇒ plant death (method used to kill undesirable trees).

Special / Unusual Cases

• Palms
  • Monocots; lack true vascular cambium.
  • Thick stems produced by a massive primary meristem (primary thickening), especially where leaf bases encircle stem.
  • Single apical meristem ⇒ damage to “cabbage” (apical bud) kills plant (hence term “millionaire’s salad”).
  • NZ example: Nīkau palm.
• Cabbage tree (Cordyline australis)
  • Monocot exhibiting anomalous secondary growth via multiple cambial zones.
  • Wood spongy, less lignified than dicot trees but allows rapid stem thickening to support branched canopy.

Key Terminology Review (with Quick Definitions)

• \text{Primary growth} – lengthening via apical meristems.
• \text{Secondary growth} – thickening via vascular & cork cambia.
• \text{Vascular cambium} – meristem producing secondary xylem/phloem.
• \text{Fusiform \& ray initials} – axial vs radial cambial cells.
• \text{Periclinal / Anticlinal divisions} – parallel / perpendicular to organ surface.
• \text{Fascicular / Interfascicular cambium} – within vs between primary bundles.
• \text{Periderm} – phellem + phellogen + phelloderm.
• \text{Bark} – all tissues outside vascular cambium.
• Section planes: transverse, radial, tangential.
• \text{Lignophyte} – plant lineage ancestrally possessing wood.

Ethical, Practical & Real-World Notes

• Bark harvesting (e.g.
  cork oak) must avoid girdling to prevent tree death; managed by removing only outer periderm.
• Ring-barking as a control technique must consider ecological impact and potential legal restrictions.
• Understanding cambial behavior aids forestry, orchard pruning, grafting techniques, and interpreting tree-ring climate data.

Connections to Previous & Upcoming Lectures

• Builds on knowledge of primary root & stem anatomy (Lectures 1 & 2).
• Sets stage for Lecture 4: leaf structure & anatomy (ties into primary growth topic of apical meristems sending out leaves).

Study Tips

• Practice drawing cross-sections of root vs stem cambium initiation.
• Memorize orientation terms (periclinal, anticlinal, radial, tangential).
• Relate annual ring patterns to environmental conditions (dendrochronology).
• Compare monocot vs eudicot strategies for support and water conduction.
• Observe local woody plants: locate phloem fibers, judge bark thickness, and attempt to spot different periderm layers.