🌿 Plant Kingdom Study Notes

3.1 Classification Systems

Early botanists grouped plants using only a few visible traits. Over time three broad approaches — and several modern tools — developed.

Newer tools

• Numerical Taxonomy — computer-based; each observable character is coded & numbered so hundreds of traits are weighed at once.
• Cytotaxonomy — based on cytological information: chromosome number, structure & behaviour.
• Chemotaxonomy — uses chemical constituents of the plant to resolve confusions.

3.2 Algae

Algae are chlorophyll-bearing, simple, thalloid (no true root–stem–leaf), largely aquatic autotrophs. The plant body is a thallus. They occur in fresh & marine water, moist stones, soil, wood, and even in/on other organisms.

Reproduction — three routes

• Vegetative — by fragmentation; each fragment grows into a new thallus.
• Asexual — mostly by zoospores (flagellated, motile) that germinate into new plants.
• Sexual — fusion of gametes:

The three classes — compare carefully

Economic & ecological importance

• Algae carry out at least half of Earth's CO₂ fixation (photosynthesis); they are primary producers of energy-rich food in aquatic food chains.
• Food: Porphyra, Laminaria, Sargassum.
• Hydrocolloids: algin (brown algae), carrageen (red algae). Agar from Gelidium & Gracilaria — culture media, jellies, ice-creams.
• Chlorella & Spirulina — protein-rich food supplements (even for space travellers).

3.3 Bryophytes

Bryophytes are the "amphibians of the plant kingdom" — they live on land but need water to complete sexual reproduction. They lack true vascular tissue (no xylem/phloem).

• The main plant body is the gametophyte (haploid, dominant, photosynthetic), attached by rhizoids.
• Male organ = antheridium → flagellated antherozoids; female organ = archegonium (flask-shaped) → one egg.
• Water lets antherozoids swim to the archegonium → zygote.
• Zygote → sporophyte (diploid, partly dependent on gametophyte) → spores by meiosis → new gametophytes.

Liverworts

Moist, shady places; thalloid body (e.g. Marchantia). Asexual reproduction by gemmae (green multicellular buds in gemma cups) or fragmentation.

Mosses

Two stages: (1) protonema — creeping, green, branched filamentous stage from a spore; (2) leafy stage — upright, spirally arranged leaves, bears sex organs. Examples: Funaria, Polytrichum, Sphagnum.

3.4 Pteridophytes

Pteridophytes are the first true land plants with vascular tissue (xylem & phloem). Examples: Selaginella, Equisetum, Pteris, Adiantum, Lycopodium, Salvinia, Marsilea.

• Dominant phase = sporophyte (2n) with true roots, stem & leaves.
• Leaves: small (microphylls, e.g. Selaginella) or large (macrophylls, ferns).
• Sporophytes bear sporangia on sporophylls; clusters of sporangia = sori.
• Sporangia → spores by meiosis → small, free-living, photosynthetic prothallus (gametophyte) needing cool, damp, shady soil.
• Gametophyte bears antheridia & archegonia; water needed for fertilisation.

Homospory vs Heterospory

3.5 Gymnosperms

Gymnosperms bear naked seeds — ovules are not enclosed in an ovary, so seeds are not covered by a fruit. Examples: Cycas, Pinus, Ginkgo, Cedrus, Ephedra.

• Small shrubs to giant trees (Sequoia, the redwood — among the tallest living organisms).
• Roots usually tap roots; Cycas has coralloid roots (N₂-fixing cyanobacteria); Pinus has mycorrhiza.
• Leaves may be needle-like with thick cuticle & sunken stomata (xerophytic adaptations).
• Heterosporous — micro- & megaspores borne on cones / strobili.
• Male cone → microsporophylls + microsporangia → pollen grains (microspores).
• Female cone → megasporophylls + ovules; megaspore → female gametophyte with archegonia.
• No water needed — wind carries pollen to the ovule; pollen tube delivers male gametes.
• Zygote → embryo; ovule → naked seed (not inside a fruit).

3.6 Angiosperms

Angiosperms = flowering plants. Seeds develop inside an ovary that matures into a fruit. Sizes range from tiny Wolffia to giant Eucalyptus (~100 m).

• Stamen (male): anther's pollen sacs → pollen grains (microspores) → male gametes.
• Pistil/carpel (female): ovary → ovules; each ovule has an embryo sac (female gametophyte) with an egg + two polar nuclei.
• Double fertilisation (unique to angiosperms): male gamete + egg → zygote (2n) (syngamy); second male gamete + two polar nuclei → primary endosperm nucleus (3n) (triple fusion) → triploid endosperm.

Two classes

3.7 Life Cycles & Alternation of Generations

Plants alternate between a haploid gametophyte (makes gametes by mitosis) and a diploid sporophyte (makes spores by meiosis). Which phase dominates defines the life-cycle type.