Lecture 19 Plant Form and Function Part 1

Be able to:
- Describe defining characteristics of monocots vs. eudicots (a.k.a. dicots).
- Explain the basic plant body plan and the origin of morphological variability.
- Recognize major cell and tissue types (dermal, vascular, ground).
- Outline nutrient-uptake strategies, especially nitrogen acquisition.

Soil
- Opaque, nutrient- and water-rich, mechanically supportive.
- Poor light → unsuitable for photosynthesis.
Air
- Translucent → abundant light for photosynthesis.
- Scarce water/nutrients, little mechanical support.
Evolutionary solution: separate organs
- Roots: acquire water, minerals, anchorage; hormonal roles.
- Shoots: photosynthesis, reproduction, mechanical elevation.

Bromeliads of the Atacama Desert
- Virtually rain-free habitat; absorb fog/mist directly through leaves.
- Possess only minimal anchoring roots.
Parasitic mistletoes
- Tap directly into host’s vascular tissue; roots reduced to haustoria.
- Visible on deciduous hosts when host foliage is absent.
Ghost orchid (Dendrophylax spp.)
- Photosynthetic green roots spread over tree bark.
- Shoot restricted to tiny apical tissue that periodically produces prized flowers.
Tristerix (parasite on cacti) & Rafflesia (Malaysian vine parasite)
- Live almost entirely as internal clusters of cells in host vascular system.
- Only visible phase = gigantic, foul-smelling flowers pollinated by carrion flies.

“Basal” angiosperm quest
- Early hypotheses: Magnoliids primitive; later focus on Queensland’s Austrobaileyales.
- Molecular phylogenetics → most basal extant lineage = Amborella trichopoda.
- Single-species family; New Caledonian rainforest shrub with small white flowers.
- Illustrates how primitive does NOT mean morphologically spectacular.
Size of lineages (approx.)
- Eudicots: \approx 300\,000 species.
- Monocots: >100\,000 species.
- Magnoliids & basal groups: far fewer (exact figure not specified).

Cotyledons (embryonic leaves)
- Monocot = 1; Eudicot = 2.
Leaf venation
- Monocot: parallel.
- Eudicot: reticulate (net-like).
Stem vascular bundle arrangement
- Monocot: scattered/complex.
- Eudicot: ring near periphery.
Root architecture
- Monocot: fibrous.
- Eudicot: dominant taproot with lateral branches.
Floral organs
- Monocot: multiples of 3.
- Eudicot: multiples of 4 or 5.
Caveats: traits are typical, not absolutely diagnostic for every species.

Core concept: Apical meristem continually lays down repeating units (modules).
- Module = internode + leaf + axillary bud (meristem) subtended by that leaf.
Fate of meristems
- Continue vegetative growth.
- Convert to reproductive growth (flower/inflorescence).
- Remain dormant.
Consequences: seemingly limitless architectural diversity arises from simple positional changes, activation/inactivation patterns, and morphological modifications of any module component.

Rhizomes: underground stems (e.g., ginger). True roots emerge from rhizome nodes.
Bulbs: very short stem with tightly packed storage leaves (e.g., onion, garlic).
Stolons/runners: above-ground or surface stems for asexual spread (e.g., strawberry).
Tubers: enlarged storage stems (e.g., potato).
- “Eyes” = axillary buds at nodes; sprout into branches when dormancy broken.

Phyllotaxy
- Alternate, opposite, decussate, whorled (up to 20–30 leaves per node).
Compound leaves vs. leaflets
- Single leaf subdivided into many leaflets; distinguished by absence of axillary buds at leaflet bases.
Functional specializations
- Carnivory
- Venus flytrap: hinged leaf that snaps shut; adaptation to N-poor soils.
- Pitcher plants: tubular leaves with digestive fluid; attract prey via nectar at rim.
  - Unique Bornean variant employs tree-top marsupials as living toilets; plant offers nectar, collects feces/urine for N.
- Climbing: tendrils (modified leaves) coil around supports.
- Defense: cactus spines = leaves transformed into hardened, sharp structures; photosynthesis shifts to green stems.

Simpler internal organization than animals; 3 main tissue systems:
1. Dermal tissue
- Epidermis + cuticle: waterproofing, pathogen barrier.
- Specialized epidermal structures: hairs, spines, stinging trichomes (e.g., nettles); stomata for gas exchange/water regulation.
1. Vascular tissue (detailed in later lectures)
- Xylem: mainly water/mineral transport.
- Phloem: mainly sugar transport.
1. Ground tissue
- Parenchyma
  - Thin-walled, living; photosynthesis (chloroplasts), storage (amyloplasts), pigmentation (chromoplasts).
- Collenchyma
  - Thickened primary walls; flexible support (e.g., celery petioles).
- Sclerenchyma
  - Lignified, dead at maturity.
  - Fibers: long, cluster together → tensile strength; dietary “fiber” in humans.
  - Sclereids: shorter protective cells; cause gritty texture in pears; seed coats.
Meristematic tissue (generative)
- Localized regions of perpetual cell division (apical, axillary, cambial, etc.).

Conventional pathway: root absorption of nitrate, ammonium, phosphate, etc.
Alternative/complementary strategies illustrated
- Atmospheric mist absorption (Atacama bromeliads).
- Haustorial parasitism (mistletoes, Rafflesia, Tristerix).
- Carnivory: ingestion of animal prey → nitrogen & micronutrients.
- Mutualistic “toilet” strategy with vertebrates (Bornean pitcher plants + marsupials).
Significance
- Highlight evolutionary flexibility when key nutrients (especially N) are limiting.
- Demonstrate ecological interactions ranging from parasitism to mutualism.

Form follows necessity: dual terrestrial environment drives bifurcated organ systems.
Modularity = evolvability: simple, repeatable units facilitate immense morphological & functional innovation.
Convergent solutions: compound leaves, carnivory, parasitism, etc. have evolved multiple times, underscoring adaptive value.
Trade-offs & economics
- Plants “pay” in easily produced resources (e.g., sugars/nectar) to gain scarce resources (e.g., nitrogen).
- Structural investment (lignin, cellulose) confers defense/support at metabolic cost.
Human relevance
- Crop structures (potato tubers, onion bulbs) are modified stems/leaves → informs breeding & storage.
- Dietary fiber = indigestible plant sclerenchyma, essential for colon health.
Ethical/Ecological insights
- Biodiversity hotspots (e.g., New Caledonia, Borneo) harbor unique evolutionary experiments; conservation maintains living laboratories.

Practice identifying modules on real plants: locate nodes, internodes, axillary buds.
Dissect common vegetables to spot modified organs (e.g., potato eyes, onion stem plate).
Use floral part counts to predict monocot vs. eudicot in the field.
Relate nutrient strategies to habitat conditions (e.g., carnivory ↔ N-poor wetlands).