The Plant Kingdom
470 Million Years of Evolution🌿 From Water to Land — The Greatest Evolutionary Challenge
Plants evolved from green algae ancestors in water. Moving onto land presented a series of challenges: drying out (desiccation), transporting water and nutrients without being surrounded by them, supporting an upright body against gravity, and reproducing without water to carry gametes. Each major plant group represents an evolutionary solution to one or more of these problems.
The plant kingdom is divided into four main groups based on key characteristics: presence of vascular tissue, presence of seeds, and whether seeds are enclosed in fruit. Each advance allowed plants to colonise drier, more challenging environments.
Bryophytes (Mosses, Liverworts, Hornworts)
No vascular tissue. No true roots, stems or leaves. Require water for reproduction (swimming sperm). Restricted to moist environments. Most primitive land plants.
Pteridophytes (Ferns, Horsetails, Club mosses)
Have vascular tissue (xylem and phloem) — can transport water and nutrients. Have true roots, stems and leaves. Still require water for reproduction. No seeds.
Gymnosperms (Conifers, Cycads, Ginkgo)
Have vascular tissue AND seeds — embryo protected in a seed with food store. Pollen eliminates need for water in reproduction. Seeds are naked (exposed on cones, not enclosed in fruit).
Angiosperms (Flowering Plants)
The most advanced and diverse group. Seeds enclosed in fruit (ovary wall). Flowers for pollination, fruit for seed dispersal. ~300 000 species — dominate most terrestrial ecosystems today.
| Feature | Bryophytes | Pteridophytes | Gymnosperms | Angiosperms |
|---|---|---|---|---|
| Vascular tissue | ❌ No | ✅ Yes | ✅ Yes | ✅ Yes |
| True roots/stems/leaves | ❌ No (rhizoids) | ✅ Yes | ✅ Yes | ✅ Yes |
| Seeds | ❌ No (spores) | ❌ No (spores) | ✅ Yes (naked) | ✅ Yes (enclosed) |
| Flowers | ❌ No | ❌ No | ❌ No | ✅ Yes |
| Fruit | ❌ No | ❌ No | ❌ No | ✅ Yes |
| Water needed for reproduction | ✅ Yes | ✅ Yes | ❌ No (pollen) | ❌ No (pollen) |
| Dominant generation | Gametophyte | Sporophyte | Sporophyte | Sporophyte |
Bryophytes
Non-vascular Plants🌱 The Pioneers of Land
Bryophytes were among the first plants to colonise land, approximately 470 million years ago. They lack vascular tissue (xylem and phloem), so they cannot transport water and nutrients efficiently over long distances — this keeps them small and restricted to moist environments. They are the simplest land plants and provide important clues about how the transition from aquatic to terrestrial life occurred.
🌿 Mosses (Musci)
- Most familiar bryophytes — small, cushion-forming, usually green
- Have leaf-like structures (not true leaves — no vascular tissue inside)
- Anchored by rhizoids — hair-like structures that absorb water; not true roots
- Absorb water over entire surface — like a sponge
- Produce spores in a capsule on a stalk (sporophyte)
- Pioneer species — colonise bare rock, start soil formation
- Example: Sphagnum moss — forms peat bogs; major carbon store
🍃 Liverworts (Hepaticae) & Hornworts
- Liverworts — flat, lobed, liver-shaped thallus lying on moist surfaces; some have leaf-like structures
- Name comes from ancient belief that liver-shaped plants treated liver disease (Doctrine of Signatures)
- Hornworts — flat thallus with horn-like sporophyte projecting upward
- Both groups lack vascular tissue, have rhizoids, require moist habitats
- Found on moist rock faces, forest floors, stream banks
🌿 Gametophyte Generation (Dominant in Bryophytes)
- The familiar green moss plant IS the gametophyte (haploid — n)
- Produces gametes (sex cells) — sperm and eggs in specialised structures
- Sperm are flagellated — MUST swim through water to reach egg
- This is why bryophytes require water for sexual reproduction
- After fertilisation, the zygote (2n) grows into the sporophyte — which remains attached to and dependent on the gametophyte
📦 Sporophyte Generation (Dependent in Bryophytes)
- The stalk-and-capsule structure growing from the moss (diploid — 2n)
- Dependent on gametophyte for nutrition
- Produces spores by meiosis inside the capsule
- Spores are haploid (n) and dispersed by wind
- Each spore can germinate into a new gametophyte plant
- In bryophytes, gametophyte = dominant; in all other plant groups, sporophyte = dominant
Pteridophytes
Vascular, Seedless Plants🌾 The First Vascular Plants
Pteridophytes (ferns, horsetails, club mosses) were the dominant land plants for much of the Carboniferous period (~360–300 million years ago). Giant tree ferns formed the vast coal swamp forests whose compressed remains became the coal we burn today. The key advance over bryophytes: vascular tissue (xylem and phloem) for efficient water and nutrient transport, allowing much larger plant bodies. They still require water for reproduction.
🏗️ Structure
- Fronds — the large divided leaves; young fronds are tightly coiled (croziers/fiddleheads) and unroll as they mature
- Rhizome — horizontal underground stem from which fronds and roots grow
- True roots — absorb water and minerals from soil
- Vascular tissue — xylem transports water; phloem transports sugars
- Sori — brown dot clusters on the underside of fronds; contain sporangia (spore-producing structures)
🔄 Reproduction
- Spores released from sori → germinate into tiny heart-shaped prothallus (the gametophyte)
- Prothallus produces sperm and eggs; sperm must swim in water to reach egg
- Fertilised egg → new fern plant (sporophyte — the dominant generation)
- Sporophyte is the large fern plant you recognise
- Gametophyte (prothallus) is tiny, short-lived, and dependent on moist conditions
🎋 Horsetails (Sphenophyta)
- Jointed, hollow, ribbed stems with whorls of scale-like leaves at joints
- Single living genus: Equisetum
- Silica in stems — historically used to scour pots ("scouring rushes")
- Ancestors were 30-metre trees in Carboniferous coal swamp forests
- Produce spores in cone-like structures at stem tips
🌿 Club Mosses (Lycophyta)
- Small, creeping plants with small scale-like leaves; look like large mosses but ARE vascular
- Example: Lycopodium (ground pine/running cedar)
- Produce spores in club-shaped cones (strobili) at shoot tips
- Ancestors (Lepidodendron) were 40-metre trees — major coal-forming plants
- Spores of Lycopodium are highly flammable — historically used in theatrical flash effects
Seed Plants
Gymnosperms & Angiosperms🌳 The Seed — Evolution's Master Invention
The seed was one of the most important innovations in plant evolution. A seed contains an embryo, a food supply (endosperm or cotyledons), and a protective coat — all in one package. Most importantly, the seed can remain dormant for months or years and survive conditions that would kill an actively growing plant. Seeds also eliminated the need for water in reproduction: pollen carries the male gametes directly to the female, replacing swimming sperm entirely.
🌲 Conifers (Coniferophyta)
- Largest gymnosperm group — pines, firs, spruces, cedars, cypresses, Podocarpus
- Bear seeds in cones (strobili) — male cones produce pollen; female cones bear seeds
- Most are evergreen with needle-like or scale-like leaves — adaptations to cold or dry climates (reduced surface area = reduced water loss)
- World's largest (giant sequoia), tallest (coast redwood), and oldest (bristlecone pine, 5 000 years) organisms are conifers
- Major timber trees worldwide
🌴 Cycads & Ginkgo
- Cycads — palm-like appearance; large compound leaves; separate male and female plants; seeds in large cones; living fossils unchanged for 280 million years; SA has Encephalartos (many critically endangered)
- Ginkgo biloba — the only surviving species of a once-diverse group; fan-shaped leaves; deciduous; extremely long-lived; survived Hiroshima atomic bomb; extracts used in herbal medicine
- Both groups are "living fossils" — little changed from their Mesozoic ancestors
🌸 Key Features
- Seeds enclosed in an ovary — the ovary wall becomes the fruit after fertilisation
- Flowers — specialised reproductive structures; attract pollinators or adapted for wind pollination
- Double fertilisation — unique to angiosperms: one sperm fertilises egg (→ embryo); another sperm fertilises central cell (→ endosperm food supply)
- Fruit — develops from ovary wall; aids seed dispersal (eaten by animals, wind-borne, hooks, explosive)
- Broadest range of habitats of any plant group
🌿 Monocots vs Dicots
- Monocots: 1 seed leaf (cotyledon); parallel leaf veins; flower parts in 3s; scattered vascular bundles; fibrous root system. Examples: grasses, lilies, orchids, palms, maize
- Dicots (Eudicots): 2 seed leaves; branching (net) veins; flower parts in 4s or 5s; ring of vascular bundles; taproot system. Examples: roses, oaks, beans, sunflowers, proteas
| Structure | Function | Male or Female? |
|---|---|---|
| Sepal (calyx) | Protects flower bud before opening; usually green | Neither (accessory) |
| Petal (corolla) | Attracts pollinators by colour, shape, scent; may guide insects to nectar | Neither (accessory) |
| Stamen (filament + anther) | Anther produces pollen (male gametophyte containing sperm cells) | ♂ Male |
| Carpel/Pistil (stigma + style + ovary) | Stigma receives pollen; style: pollen tube grows down; ovary contains ovules (eggs) | ♀ Female |
| Nectary | Produces nectar — rewards pollinators (bees, butterflies, birds) | Neither (accessory) |
| Receptacle | Base of flower — all parts attached here; swells to form fruit in some species | Neither |
South African Plant Biodiversity
One of the World's Richest Floras🇿🇦 The Cape Floral Kingdom — A World Treasure
South Africa is one of the world's six floral kingdoms, despite covering only 0.04% of Earth's land surface. The Cape Floristic Region (CFR) contains over 9 000 plant species — more than the entire British Isles. It is a UNESCO World Heritage Site and a global biodiversity hotspot. More than 69% of its plant species are found nowhere else on Earth (endemic). Understanding this extraordinary diversity — and the threats it faces — is essential content for South African Life Sciences.
🌿 The Three Fynbos Families
- Proteaceae — proteas, leucadendrons, leucospermums (pincushions); iconic large flowers; seeds dispersed by ants (myrmecochory) or stored in cones released by fire; Protea cynaroides (king protea) = SA national flower
- Ericaceae — ericas (heaths); over 860 species in SA, 99% endemic; small, tubular flowers adapted for sunbird and fly pollination
- Restionaceae — restios (reeds); rush-like plants with photosynthetic stems; reduced or absent leaves; wind-pollinated
🔥 Fire Adaptation
- Fynbos is a fire-adapted ecosystem — fire is essential, not destructive
- Many proteas are serotinous — seeds stored in woody cones, only released after fire heat
- Many bulbous plants (geophytes) survive as underground bulbs during fire, re-sprouting rapidly after
- Smoke triggers germination of many seeds (smoke-related germination cues)
- Without periodic fire: shrubs senesce (age), fuel load builds, alien invasives dominate
- Optimal fire return interval: 10–15 years
| Biome | Location | Key Plants | Adaptations |
|---|---|---|---|
| Succulent Karoo | West coast, Namaqualand | Mesembs (vygies), euphorbias, aloes, stone plants (Lithops) | CAM photosynthesis; water storage in thick succulent leaves/stems; shallow roots for capturing light rain |
| Nama-Karoo | Central plateau | Dwarf shrubs, grasses, Karoo bushes | Drought tolerance; many annual plants germinate only after rain |
| Grassland | Highveld, Drakensberg | Grasses (Themeda triandra), geophytes (bulbs), proteas in escarpment | Underground storage organs survive winter frost and summer fires; grasses have meristems at base not tip |
| Savanna | Bushveld, Lowveld, KZN | Marula (Sclerocarya birrea), mopane (Colophospermum mopane), acacia/thorn trees (Vachellia) | Deep roots for dry season water; thorns and tannins against browsing; fire-resistant bark |
| Forests | KZN coast, Eastern Cape, Knysna | Yellowwood (Podocarpus), stinkwood (Ocotea bullata), wild fig | Shade-tolerant understorey species; closed canopy limits fire; high rainfall reduces drought stress |
🎯 Plant Kingdom Assessment
Eight questions on plant biodiversity and classification.