Kingdom Plantae — Comprehensive Study Notes

Plantae – General Characteristics

• Autotrophic organisms: perform photosynthesis to synthesize their own food.
• Habitats: terrestrial (land) and aquatic environments.
• Cell walls: composed of cellulose (polysaccharide chains of \beta-glucose).
• Chloroplasts present for photosynthesis.
• Presumed ancestors → green algae; shared traits:
  • Photosynthetic pigment: chlorophyll a and b.
  • Cell wall material: cellulose.
  • Food reserve: starch.
  • Cytokinesis: formation of a cell plate.

Innovations Distinguishing Plants From Green Algae

• Waxy cuticle on aerial surfaces minimizes desiccation.
• Stomata on leaves regulate \text{CO}_2 uptake & water loss.
• Leaves specialized for carbon fixation (increased surface area, vascular support).
• Multicellular gametangia with sterile jackets protect gametes.
• Zygote develops inside female gametangium (embryophyte condition).
• Alternation of generations:
  • Multicellular haploid phase = gametophyte (produces gametes).
  • Multicellular diploid phase = sporophyte (produces spores).
  • In most modern plants the sporophyte is dominant.

Alternation of Generations – Core Life-Cycle Steps

• Gametophyte (n) → produces antheridia (sperm) &/or archegonia (egg).
• Fertilization n + n \;\Rightarrow\; 2n produces zygote.
• Zygote mitotically forms embryo protected in archegonium.
• Embryo grows into mature sporophyte (2n).
• Sporogenous (spore-mother) cells undergo meiosis 2n \to n → spores.
• Spores mitose → new gametophytes.

Classification Overview (Modern Major Lineages)

• Non-vascular (Bryophytes):
  • Bryophyta – true mosses.
  • Hepaticophyta – liverworts.
  • Anthocerotophyta – hornworts.
• Seedless vascular:
  • Pteridophyta (true ferns).
  • Psilotophyta (whisk ferns).
  • Sphenophyta (horsetails).
  • Lycophyta (club & spike mosses).
• Seed vascular:
  • Gymnosperms → Coniferophyta, Cycadophyta, Ginkgophyta, Gnetophyta.
  • Angiosperms (Anthophyta) → monocots & dicots.

Non-Vascular Plants (Bryophytes)

Bryophyta – True Mosses

• Moist habitats; form dense colonies.
• Plant body: rhizoids + stem-like axis with leaf-like blades; no vascular tissue.
• Dominant phase: gametophyte (n).
• Reproductive organs:
  • Antheridia → flagellated sperm.
  • Archegonia → single immotile egg.
• Fertilization water-dependent; zygote (2n) remains on female gametophyte.
• Dependent sporophyte components:
  • Foot – anchors to gametophyte.
  • Seta – stalk.
  • Capsule – contains sporogenous cells; capped by calyptra.
• Meiosis inside capsule → spores → protonema → new gametophytes.
• Ecological / economic roles:
  • Soil stabilization, erosion control.
  • Micro-habitats for invertebrates.
  • Phenolic-rich walls slow decay → natural preservative (e.g.
    bog mummies in Sphagnum peat).
  • Water & mineral retention → soil conditioner.
  • Peat (Sphagnum) harvested for fuel.
• Representative genera: Sphagnum, Polytrichum commune, Funaria, Bryum.

Hepaticophyta – Liverworts

• Strictly damp habitats; dorsiventrally flattened thallus (one cell layer, undifferentiated).
• Sexual reproduction:
  • On separate thalli: antheridiophores (bearing antheridia) vs archegoniophores (bearing archegonia).
  • Rain splash aids sperm transfer.
  • Zygote \to sporophyte that stays attached to female thallus; sporogenous cells undergo meiosis → spores.
• Asexual reproduction: gemmae cups produce tiny gemmae; dispersed by rain/animals → new thalli.
• Example: Marchantia polymorpha.

Anthocerotophyta – Hornworts

• Sporophyte resembles a green horn/grass blade \approx 5\,\text{cm} tall; splits from tip to release spores.
• Gametophyte: 1–2\,\text{cm} wide thallus; can bear multiple sporophytes.
• Internal sporophyte features: columella, pseudo-elaters.
• Example: Anthoceros spp.

Seedless Vascular Plants

Key Adaptations for Terrestrial Life

• True roots: anchorage + water/mineral absorption → allowed taller shoots.
• Leaves:
  • Microphylls = small, single vein.
  • Megaphylls = larger, multi-veined → increased photosynthetic area.
• Sporophylls = modified leaves bearing sporangia.
  • Ferns → sori; Lycophytes → cones (strobili).
• Spore types:
  • Homosporous: one spore kind → bisexual gametophyte.
  • Heterosporous: microspores (♂) + megaspores (♀) → precursor to seeds.

Pteridophyta – True Ferns

• Possess megaphyllous fronds; horizontal rhizomes.
• Dominant sporophyte independent of gametophyte.
• Mostly homosporous.
• Life-cycle notes:
  1. Meiosis in sori 2n \to n produces spores.
  2. Spores → heart-shaped prothallus (gametophyte) with rhizoids.
  3. Prothallus bears both antheridia & archegonia; cross-fertilization common.
  4. Zygote → young sporophyte on gametophyte; gametophyte dies.

Psilotophyta – Whisk Ferns (Psilotum)

• Lacking true roots & leaves; dichotomously branched photosynthetic stems.
• Underground mycorrhizal prothalli (non-photosynthetic).
• Aerial stems bear fused sporangia → homospores.

Sphenophyta – Horsetails (Equisetum)

• Wet/marshy habitats; heights up to 1.3\,\text{m}.
• Jointed, silica-impregnated hollow stems; reduced megaphylls in whorls.
• Strobili at stem tips; homosporous; life cycle like ferns.
• Both generations photosynthetic & independent at maturity.

Lycophyta – Club & Spike Mosses

• True roots, rhizomes, microphylls; strobili at shoot tips.
• Lycopodium – homosporous club mosses.
• Selaginella – heterosporous spike mosses:
  • Microsporangia → microspores → male gametophytes (antheridia).
  • Megasporangia → megaspores → female gametophytes (archegonia).
  • Heterospory considered evolutionary precursor to seed habit.

Seed Plants – Generalities

• Seed vs spore:
  • Seed houses multicellular embryo (root, shoot, cotyledons) + food store + protective coat.
  • Spore = single cell with minimal reserves.
• Seeds develop from ovules (megasporangium + integuments) after fertilization.
• All seed plants are heterosporous; reduced gametophytes depend on sporophyte.

Gymnosperms – “Naked Seeds”

Coniferophyta – Conifers (spruce, fir, pine, cedar)

• Woody trees/shrubs; secondary xylem (tracheids) added annually.
• Resin ducts deter fungi/insects.
• Leaf morphologies: needle-, scale-, awl-like.
• Mostly monoecious; separate male & female cones.

Male (Pollen) Cone

• Microsporophylls bear 2 microsporangia each.
• Microsporocytes 2n → meiosis → 4 microspores n.
• Microspore develops into 4-celled pollen grain with two air sacs → wind dispersal.

Female (Seed) Cone

• Cone scale (sporophyll) bears 2 ovules (megasporangia).
• Megasporocyte 2n → meiosis → 4 megaspores; 1 functional → female gametophyte with archegonia.

Pollination & Fertilization

1. Wind-borne pollen adheres to female cone; germinates pollen tube through nucellus.
2. Generative cell mitoses → two non-motile sperm; one fertilizes egg n + n \to 2n embryo.
3. Seed structure:
   • Embryo (sporophyte, 2n).
   • Female gametophyte tissue (haploid food reserve).
   • Seed coat (diploid integument) often winged for wind dispersal.

Life-Cycle Synopsis

• Dominant sporophyte; microscopic gametophytes within cones; sperm delivered via pollen tube (no free water required).

Ecological / Human Importance

• Habitat & food for fauna; roots resist erosion; timber & paper; ornamental uses.

Cycadophyta – Cycads

• Palm-like appearance; tropical/subtropical; dioecious.
• Motile, flagellated sperm (primitive trait) swim within pollen tube.

Ginkgophyta – Ginkgo biloba

• Only extant species; dioecious; motile sperm; seeds exposed, fleshy coat smells foul.
• Medicinal uses: extracts may improve cerebral blood flow & memory (Alzheimer’s studies).

Gnetophyta – Gnetum, Ephedra, Welwitschia

• Possess vessel elements (advanced xylem like angiosperms).
• Cone clusters resemble flowers; life-cycle details parallel angiosperms.
• Ephedra → source of ephedrine (cardiac stimulant).
• Welwitschia → desert species with two persistent strap-like leaves.

Angiosperms – Phylum Anthophyta (Flowering Plants)

Monocots vs Dicots

Flower Structure & Terminology

• Stamen (male) = filament + anther (microsporangia).
• Carpel/Pistil (female) = stigma + style + ovary (contains ovules).
• Perianth: sepals (calyx) + petals (corolla).
• Perfect flower: both male & female parts; imperfect: one sex only.
• Pollination vectors: wind, insects, birds, mammals.

Gametophytes Within Flower

• Inside anther:
  1. Microsporocytes 2n → meiosis → tetrad of 4 microspores n.
  2. Each microspore mitoses → pollen grain (tube cell + generative cell).
• Inside ovule (megasporangium):
  1. Megasporocyte 2n → meiosis → 4 megaspores; 3 degenerate.
  2. Surviving megaspore undergoes 3 mitoses → 8-nucleate embryo sac (megagametophyte):
  • 1 egg cell, 2 synergids, 2 polar nuclei (central cell), 3 antipodals.

Double Fertilization (Unique to Angiosperms)

1. Pollen grain on stigma germinates; tube cell forms pollen tube down style.
2. Generative cell divides → 2 sperm (n each).
3. At micropyle, pollen tube releases sperm:
   • Sperm 1 + egg → zygote 2n.
   • Sperm 2 + 2 polar nuclei → endosperm 3n.
4. Results: embryo nourished by triploid endosperm.

Seed & Fruit Development

• Embryo develops within embryo sac; integuments + nucellus + sac wall → seed coat.
• Ovary wall matures into fruit (pericarp); fruit type influences seed dispersal mechanism.

Comparative Summary: Spores vs Seeds

• Spores: single haploid cell, minimal reserves, thin wall.
• Seeds: multicellular 2n embryo, food supply, thick coat; survive long dormancy.

Closing Notes

• Evolutionary trend: dominance shift from gametophyte (bryophytes) → sporophyte (vascular plants).
• Progressive reduction of gametophyte size & dependency within sporophyte tissues (extreme in seeds).
• Transitions: homospory → heterospory → seed habit; water-dependent fertilization → pollen tube delivery.
• Practical relevance: ecology (soil, habitats), economy (timber, fuel, medicine), and evolutionary insights into terrestrial adaptation.