Plant Secondary Growth – Vocabulary Review

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 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.

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.

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.

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 (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.

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 = 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).

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.

\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.

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.

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).

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.