Plant Kingdom

Introduction

Kingdom Plantae encompasses all multicellular, eukaryotic, photosynthetic organisms with cell walls of cellulose. The plant kingdom is classified into major groups based on the presence or absence of vascular tissue, seeds, and flowers. All plants show alternation of generations — a life cycle alternating between a haploid gametophyte phase and a diploid sporophyte phase.

Major Groups of Plants

1. Algae (Cryptogamae — non-vascular, no seeds)

Algae are simple, largely aquatic plants without true roots, stems, or leaves. They range from unicellular ( · Chlamydomonas · ) to multicellular ( · Kelp · ). Classified by pigment:
Chlorophyceae (Green algae): chlorophyll a and b; store starch; · Spirogyra · , · Volvox · , · Ulva · . Pyrenoids are protein bodies associated with starch synthesis.
Phaeophyceae (Brown algae): fucoxanthin pigment; mannitol and laminarin as food reserves; · Ectocarpus · , · Fucus · , · Laminaria · , · Sargassum · . Show most complex thallus among algae.
Rhodophyceae (Red algae): phycoerythrin pigment; found in deep sea (can absorb blue-green light); · Polysiphonia · , · Porphyra · , · Gelidium · (agar-agar source).

2. Bryophytes (Amphibians of plant kingdom)

First truly terrestrial plants but need water for reproduction.
No vascular tissue; body is a thallus or has leaf-like and stem-like structures.
Dominant phase is gametophyte (haploid); sporophyte is dependent on gametophyte.
Liverworts ( · Marchantia · ): dorsiventrally flattened thallus; gemma cups for vegetative reproduction.
Mosses ( · Funaria · , · Polytrichum · , · Sphagnum · ): protonema stage; upright leafy gametophyte; · Sphagnum · is peat moss used as fuel and packing material.
Ecological role: prevent soil erosion; · Sphagnum · retains water.

3. Pteridophytes (First vascular plants)

Have true vascular tissue (xylem and phloem) but no seeds.
Dominant phase is sporophyte (diploid); gametophyte (prothallus) is small, independent, and needs water for fertilisation.
Spores germinate into prothallus (free-living gametophyte).
Classification based on leaf type and sporangia arrangement: · Psilotum · , · Selaginella · , · Equisetum · , · Dryopteris · (fern).
Heterospory: production of two types of spores — microspores (male) and megaspores (female). Seen in · Selaginella · and · Salvinia · . This is considered a precursor to the seed habit.
Stele: arrangement of vascular tissue in stem; important in pteridophyte taxonomy.

4. Gymnosperms (Naked-seed plants)

Have seeds but no fruits (ovules/seeds are not enclosed in an ovary/fruit).
Dominant phase is sporophyte; gametophyte is highly reduced and dependent.
Heterosporous: microspores in microsporangia (pollen sacs) and megaspores in megasporangia.
Pollen tube delivers male gametes — water not needed for fertilisation (siphonogamy).
Cycas: dioecious (separate male and female plants); large pinnate leaves; coralloid roots contain nitrogen-fixing cyanobacteria ( · Nostoc · , · Anabaena · ).
Pinus: monoecious; needle-like leaves; two types of cones (male strobili, female megastrobili); resin canals present; wood is a gymnosperm (soft wood).
Gnetum: broad net-veined leaves; angiosperm-like characters.
Sequoia ( · Sequoia sempervirens · ): tallest gymnosperm; redwood.

5. Angiosperms (Flowering plants — Covered-seed plants)

Most evolved and diverse plant group; ovules enclosed in ovary (which becomes the fruit).
Monocots: one cotyledon; parallel venation; fibrous roots; floral parts in multiples of 3; · Wheat · , · Maize · , · Grass · , · Banana · .
Dicots: two cotyledons; reticulate venation; tap root; floral parts in multiples of 4 or 5; · Pea · , · Mango · , · Rose · , · Sunflower · .
Male gametophyte = pollen grain (3 cells); Female gametophyte = embryo sac (7 cells, 8 nuclei — the Polygonum type).
Show double fertilisation (unique to angiosperms): one sperm fuses with egg (zygote → embryo); another sperm fuses with two polar nuclei (triple fusion → endosperm 3n).

Alternation of Generations

All plants alternate between haploid (n) gametophyte and diploid (2n) sporophyte generations.
In bryophytes: gametophyte dominant; sporophyte dependent.
In pteridophytes: sporophyte dominant; gametophyte (prothallus) independent.
In gymnosperms and angiosperms: sporophyte dominant; gametophyte highly reduced and dependent.

Common mistakes

Algae are NOT bryophytes; they are classified separately and lack embryo formation.
Gymnosperms have naked seeds (not in fruit); angiosperms have seeds enclosed in fruit.
In mosses, the leafy green plant you see is the gametophyte (haploid), not the sporophyte.
Double fertilisation occurs ONLY in angiosperms, not gymnosperms.

Summary

Plants are classified as algae, bryophytes, pteridophytes, gymnosperms, and angiosperms based on vascular tissue, seed production, and flower/fruit formation.
Gametophyte dominance decreases from bryophytes to angiosperms; sporophyte dominance increases.
Heterospory in pteridophytes is a stepping stone to the seed habit.
Angiosperms show double fertilisation, producing embryo (2n) and endosperm (3n).