Lecture 19 Plant Form and Function Part 2

Primary vs. Secondary Growth

Primary (Apical) Growth
- Driven by apical meristems.
- Adds length only; cells behind apex are as old as when the apex occupied that position.
- Typical of non-woody (herbaceous) plants—e.g.
- “Little herb on a mountainside” example.
- Hollywood misconception ("Jack the Giant Slayer" trailer):
- If you jump on a fast-growing stem you do not rise with it because only the tip elongates.
Secondary Growth
- Adds girth (diameter) to stems and roots.
- Produces wood (secondary xylem) and bark (secondary phloem + cork).
- Enables formation of thick trunks, indefinitely large crowns & root systems.
- Absent or highly modified in monocots.

Vascular Bundles (Primary)
- Arrangement: ring around the stem perimeter.
- Each bundle: outer phloem & inner xylem; capped externally by lignified sclerenchyma fibers ("sclerenchyma caps").
- Bundles collectively form a mechanically optimal hollow cylinder—stronger than a solid rod of the same mass.
Vascular Cambium
- Second meristem type (after apical meristem).
- Originates between primary xylem & phloem and in the interfascicular regions, joining to form a continuous ring.
- Cell fates
- Division toward inside → secondary xylem.
- Division toward outside → secondary phloem.
- Occasional anticlinal division → new cambial initials (ring expands with radius).
- Latin root "cambīre" = to change.
Cork Cambium (Phellogen)
- Third meristem; arises irregularly in peripheral cortex.
- Produces cork (phellem) outward and phelloderm inward.
- Creates protective, often fissured bark; less critical to total diameter than vascular cambium.
Wood vs. Bark
- Nearly all material inside the cambial ring = secondary xylem (wood).
- Secondary phloem is comparatively thin, crushed & sloughed off with cork during growth → explains bark shedding.
Practical implication
- Ring-barking (girdling): removal of bark + cambium kills tree by halting phloem transport; rabbits or homeowners exploit this.

Continual addition of conducting & supporting tissue → proportional expansion of:
- Leaf crown (photosynthetic capacity).
- Root network (water & nutrient uptake).
Explains transition from sapling to multi-meter-girth forest giants.

No continuous vascular cambium because bundles are scattered rather than ring-organized.
Support solutions:
- Peripheral concentration of bundles with thick sclerenchyma sheaths.
- External cylinder of fibers beneath epidermis.
- Prop / adventitious roots (e.g.
  pandanus) add buttressing & uptake area.
Consequences:
- Stem diameter essentially fixed once formed; canopy size therefore constant.
- “Bark” is mainly persistent leaf bases & scars, not true cork.
- Yet structures can be strong: palms up to 40\,–\,50\;\text{m} tall; bamboo renowned as construction material.

Field studying environmental history via tree rings.
Formation mechanism in climates with pronounced seasons:
- Spring/early summer (wet): cambium lays down large-diameter xylem vessels → light-colored “earlywood”.
- Late summer (drier): smaller vessels, thicker walls → dark “latewood”.
- Winter: cambial dormancy.
Each earlywood + latewood pair = one year → dating via increment corer without felling tree.
Applications: climate change, growth rates, disturbance events.

Regions of a Growing Root
- Root cap: protective, gravity-sensing (statolith) tissue.
- Apical meristem.
- Zone of elongation.
- Zone of maturation with root hairs (+ mucilage) → main absorption interface.
Stele (Vascular Cylinder)
- Central, not peripheral (opposite of stems).
- Layers (outside → inside): endodermis (with Casparian strip) → pericycle → xylem (+ phloem in between arms).
- Mechanical rationale:
- Roots face tensile/stretch forces (soil shrinking/swelling) more than bending; central “cable” best resists tension (elevator analogy).
- Physiological rationale:
- Endodermis/Casparian strip forces solutes through a selectively permeable membrane → first major checkpoint blocking pathogens + regulating ion entry.
Branching
- Initiated from pericycle; lateral roots erupt outward through cortex & epidermis.
Root Hairs
- Single-celled, short-lived, fragile, non-elongating → inherently poor explorers; plants offset this via symbioses.

Mycorrhizae (\approx 95 % of plant families)
- Fungal hyphae exponentially increase absorptive surface area & soil volume explored.
- Plant supplies sugars; fungus supplies water & minerals (especially \text{P} & micronutrients).
- Image example: pine seedling with vast ectomycorrhizal network (fluorescent-labeled).
- Dramatic growth gains demonstrated in soybean trials (with vs. without fungus).
Rhizobial / Actinorhizal Nitrogen Fixation
- Common in Fabaceae (peas, soy, acacias).
- Root nodules house bacteria that convert atmospheric \text{N}2 to biologically usable \mathrm{NH3} or \mathrm{NH_4^+} (energy-intensive).
- Plant trades carbohydrates for fixed nitrogen, boosting soil fertility & reducing fertilizer needs.
Proteoid (Cluster) Roots in Proteaceae
- Adaptation to P-impoverished Australian soils.
- Dense root clusters release \mathrm{H^+} via water–oxygen dissociation → locally acidify rhizosphere.
- Low pH breaks insoluble P–compound bonds, increasing \text{PO}_4^{3-} availability.

Misrepresentation of plant growth in media affects public understanding (Jack & the Beanstalk example).
Forestry & horticulture practices (girdling, pruning) hinge on cambial physiology.
Dendrochronological archives inform past climate reconstructions, wildfire history, and resource management.
Sustainable agriculture leverages symbioses (mycorrhizae, rhizobia) to reduce chemical inputs.