Plant Kingdom

🎯 Key Points

• Order of complexity: Algae → Bryophyta (first land plants, need water for fertilisation) → Pteridophyta (first vascular, no seeds) → Gymnosperms (naked seeds) → Angiosperms (seeds in fruit)
• Bryophytes called "amphibians of the plant kingdom" - live on land but need water to reproduce, just like real amphibians
• Alternation of generations: Sporophyte(2n)→spores→Gametophyte(n)→gametes→Sporophyte(2n); gametophyte dominant in bryophytes, sporophyte dominant in pteridophytes/gymnosperms/angiosperms
• Monocots: 1 cotyledon, parallel veins, fibrous roots, floral parts in 3s; Dicots: 2 cotyledons, reticulate veins, tap root, floral parts in 4s/5s
• Gymnosperms have NO double fertilisation (unlike angiosperms) and seeds remain exposed, not enclosed in fruit

Major Plant Groups

• Algae (Thallophyta): no true roots/stem/leaves; aquatic; Spirogyra, Chara, Ulva
• Bryophyta: first land plants; no vascular tissue; amphibians of plant kingdom; mosses (Funaria), liverworts (Marchantia)
• Pteridophyta: first vascular plants; no seeds; ferns (Dryopteris), Selaginella, Equisetum
• Gymnosperms: naked seeds (not in fruit); cones; Cycas, Pinus, Ginkgo
• Angiosperms: enclosed seeds (in fruit); flowering plants; most diverse group

Angiosperms: Monocots vs Dicots

Alternation of Generations

• Sporophyte (2n) → spores → Gametophyte (n) → gametes → fertilization → Sporophyte (2n)
• Bryophytes: dominant gametophyte; Pteridophytes: sporophyte dominant; Angiosperms: sporophyte dominant, gametophyte reduced

Viruses (not plants, but important)

• Non-cellular; only replicate inside host; contain DNA or RNA (not both)
• Tobacco Mosaic Virus (TMV): first virus discovered; RNA virus
• Bacteriophage: virus that infects bacteria

Classification of Algae

• Chlorophyceae (green algae): grass green due to dominant chlorophyll a and b; cell wall of cellulose; store food as starch; e.g. Chlamydomonas, Volvox, Ulva, Spirogyra, Chara
• Phaeophyceae (brown algae): marine, brown due to fucoxanthin; store food as mannitol/laminarin; cell wall has cellulose and algin; e.g. Ectocarpus, Dictyota, Laminaria, Sargassum
• Rhodophyceae (red algae): marine, red due to r-phycoerythrin pigment; store food as floridean starch; e.g. Polysiphonia, Porphyra, Gracilaria
• Economic importance: agar (from Gelidium and Gracilaria) used in labs and the food industry; algin and carrageen (from brown/red algae) used as thickeners; Chlorella and Spirulina used as protein supplements (single-cell protein)

Classification of Gymnosperms and Their Reproduction

• Gymnosperms (naked-seeded plants) include both medium-sized trees and tall trees; examples span Cycas, Pinus, Ginkgo, and Sequoia (one of the tallest trees)
• Plants are heterosporous, producing haploid microspores (male) and megaspores (female) in male and female cones (strobili) respectively; the pollen grain is carried by wind to the female cone (anemophily)
• After pollination, the pollen tube delivers two male gametes to the egg inside the ovule; one fuses with the egg, the other degenerates (no double fertilisation, unlike angiosperms); the resulting zygote develops into an embryo within the seed, but the seed remains exposed (not enclosed in a fruit)

Economic Importance of Major Plant Groups

• Bryophytes: Sphagnum (peat moss) used as a fuel and packing material for trans-shipment of living material due to its water-holding capacity; some mosses used as packing material
• Pteridophytes: some ferns used as ornamentals; Dryopteris and Lycopodium have medicinal uses
• Gymnosperms: Pinus and other conifers are major sources of timber, resin, and turpentine; Cycas seeds used in some traditional foods after detoxification
• Angiosperms: source of nearly all food crops, fibres, medicines, and ornamentals on which human civilisation depends; represent the most successful and dominant group of present-day plants

Bryophytes in Detail (Liverworts and Mosses)

• Bryophytes are the amphibians of the plant kingdom - they live in soil but need water for sexual reproduction (sperms swim to the egg); they lack true roots, stems, and leaves (have root-like rhizoids, stem-like, and leaf-like structures)
• The plant body is a gametophyte (dominant, haploid, photosynthetic); the sex organs are the male antheridium (produces biflagellate antherozoids) and female archegonium (flask-shaped, produces a single egg)
• The sporophyte is not free-living but attached to and partly dependent on the photosynthetic gametophyte; it produces spores by meiosis inside a capsule
• Liverworts (e.g. Marchantia): grow in moist, shady places; thalloid body; asexual reproduction by fragmentation or by specialised buds called gemmae formed in gemma cups
• Mosses (e.g. Funaria, Sphagnum, Polytrichum): the gametophyte has two stages - a creeping green branched protonema (from spore) and the leafy erect stage bearing sex organs
• Ecological/economic role: mosses form dense mats reducing soil erosion; Sphagnum (peat moss) provides peat (fuel) and has great water-retention capacity used in packing/horticulture

Pteridophytes in Detail

• Pteridophytes are the first true land plants with vascular tissue (xylem and phloem); they have well-differentiated true roots, stems, and leaves; found in cool, damp, shady places
• The dominant plant body is the sporophyte (diploid), which bears sporangia (usually on leaf-like sporophylls) that produce spores by meiosis; spores germinate into a small, free-living prothallus (the multicellular, photosynthetic gametophyte)
• The gametophyte needs cool, damp, shady places to grow and requires water for the swimming male gametes to reach the female gamete - restricting pteridophyte distribution to narrow, moist habitats
• Most are homosporous (produce one kind of spore); a few (Selaginella, Salvinia) are heterosporous, producing small microspores and large megaspores - considered the beginning of the seed habit
• Classified into Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum), and Pteropsida (Dryopteris, Pteris, Adiantum - the ferns)

Life Cycle Patterns (Haplontic, Diplontic, Haplo-diplontic)

• Haplontic: the dominant, photosynthetic phase is the free-living haploid gametophyte; the diploid sporophyte is represented only by the single-celled zygote, which immediately undergoes meiosis; seen in most algae (e.g. Spirogyra, Chlamydomonas)
• Diplontic: the dominant, photosynthetic phase is the diploid sporophyte; the haploid gametophyte is very reduced and represented only by gametes (or a few celled structure); seen in seed plants (gymnosperms and angiosperms) and a few algae (Fucus)
• Haplo-diplontic: both a multicellular haploid gametophyte AND a multicellular diploid sporophyte alternate - a true intermediate; in bryophytes the gametophyte is dominant, in pteridophytes the sporophyte is dominant; certain algae (Ectocarpus, Kelps) also show this pattern
• Exam clue: identify the pattern by asking which multicellular phase is dominant/free-living - dominant haploid = haplontic, dominant diploid = diplontic, both multicellular = haplo-diplontic

Algae: General Features and Reproduction

• Algae are chlorophyll-bearing, simple, thalloid, autotrophic organisms; largely aquatic (both fresh water and marine), but also found in moist stones, soil, and wood, or in association with fungi (lichens) and animals (e.g. on sloth hair)
• Form and size vary widely: colonial (Volvox), filamentous (Ulothrix, Spirogyra), and massive marine forms called kelps (up to a few metres long)
• Vegetative reproduction: by fragmentation, each fragment developing into a new thallus
• Asexual reproduction: mainly by the production of flagellated, motile zoospores that germinate into new plants
• Sexual reproduction shows three patterns: isogamous (fusion of two similar-sized gametes, flagellated as in Ulothrix or non-flagellated as in Spirogyra); anisogamous (fusion of two dissimilar-sized gametes, e.g. Eudorina); and oogamous (fusion of a large non-motile female gamete with a small motile male gamete, e.g. Volvox, Fucus)
• Ecologically vital: algae carry out at least half of the total carbon dioxide fixation on Earth by photosynthesis, acting as primary producers of the energy-rich food chain of aquatic animals

Gymnosperms: General Characteristics

• Gymnosperms bear naked ovules (not enclosed within an ovary wall), so the seeds that develop remain exposed; plants range from medium-sized trees and shrubs to the giant redwood Sequoia (one of the tallest trees)
• Roots: generally a tap root system; the roots of some genera have fungal (mycorrhizal) associations (Pinus), while in Cycas specialised coralloid roots harbour nitrogen-fixing cyanobacteria (Nostoc/Anabaena)
• Leaves are well adapted to withstand extremes of temperature, humidity, and wind - in conifers the needle-like leaves reduce surface area, and a thick cuticle with sunken stomata further check water loss
• The plants are heterosporous, producing haploid microspores and megaspores borne on distinct male and female cones (strobili); the male gametophyte and female gametophyte are retained within the parent sporophyte and are not free-living
• Vascular tissue is simpler than in angiosperms: the xylem lacks vessels (has only tracheids, except in the order Gnetales) and the phloem lacks companion cells and sieve tubes (has sieve cells instead)

Angiosperms: General Features and Double Fertilisation

• Angiosperms (flowering plants) bear seeds enclosed inside fruits; they range from the tiny Wolffia to tall trees such as Eucalyptus; they are the most diverse and dominant group of land plants
• The male sex organ (stamen) produces the pollen grain - the highly reduced male gametophyte carrying two male gametes; the ovule within the ovary contains the embryo sac - the female gametophyte, typically 7-celled and 8-nucleate, with an egg cell and a central cell bearing two polar nuclei
• Double fertilisation (unique to angiosperms): both male gametes released by the pollen tube fuse inside the embryo sac - one male gamete fuses with the egg (syngamy) to form the diploid zygote (2n) that becomes the embryo; the other fuses with the two polar nuclei (triple fusion) to form the triploid primary endosperm nucleus (3n) that develops into the nutritive endosperm
• Because two fusion events occur, the process is called double fertilisation; it ensures that endosperm (food for the embryo) develops only after successful fertilisation, avoiding waste of resources
• Angiosperms are divided into two classes - dicotyledons (two cotyledons) and monocotyledons (one cotyledon)

🚀 NEET Advanced Edge

Heterospory and seed habit evolution: Some pteridophytes (e.g. Selaginella) are heterosporous (produce both micro- and megaspores), a trait that's considered an evolutionary precursor to the seed habit seen in gymnosperms and angiosperms - a frequently tested "missing link" concept.

Why gymnosperm pollination is always wind-mediated: Gymnosperms lack the colourful flowers/nectar that attract pollinators, so they rely entirely on anemophily (wind pollination), producing vast quantities of light, winged pollen to compensate for the inefficiency - contrast this with the more targeted (and lower pollen waste) animal pollination common in angiosperms.

Distinguishing algae pigments by exam clue: "Marine, brown, stores food as laminarin" → Phaeophyceae; "Marine, red, stores floridean starch" → Rhodophyceae; "Grass-green, stores starch, fresh OR marine water" → Chlorophyceae - pigment + storage product + habitat together uniquely identify the group.

Systems of Classification

• Artificial system (Linnaeus): based on one or few vegetative and sexual characters like stamens; gave equal weight to reproductive and vegetative traits.
• Natural system (Bentham and Hooker): based on natural affinities using many characters including internal features like anatomy, embryology and phytochemistry.
• Phylogenetic system: based on evolutionary relationships, assuming organisms of the same taxa share a common ancestor.
• Numerical taxonomy uses computers; all characters are given equal importance and hundreds of characters are considered.
• Cytotaxonomy uses cytological information (chromosome number, structure, behaviour); chemotaxonomy uses chemical constituents.

Chlorophyceae, Phaeophyceae and Rhodophyceae Compared

• Chlorophyceae (green algae): chlorophyll a and b, grass-green; stored food is starch; cell wall of cellulose; examples Chlamydomonas, Volvox, Spirogyra, Ulothrix, Chara.
• Phaeophyceae (brown algae): chlorophyll a, c and fucoxanthin (brown colour); stored food laminarin or mannitol; cell wall has cellulose and algin; examples Ectocarpus, Laminaria, Fucus, Sargassum, Dictyota.
• Rhodophyceae (red algae): chlorophyll a, d and r-phycoerythrin (red colour); stored food floridean starch; mostly marine, found at greater depths; examples Polysiphonia, Porphyra, Gracilaria, Gelidium.
• Brown algae have two flagella that are unequal and laterally attached; green algae usually have two to eight equal apical flagella; red algae lack flagella.
• Agar comes from Gelidium and Gracilaria; food-grade algae include Porphyra and Laminaria.

Economic Importance of Algae

• Algae carry out about half of the total carbon dioxide fixation (photosynthesis) on Earth and increase the dissolved oxygen in their environment.
• Agar (from Gelidium and Gracilaria) is used to grow microbes and in making ice-creams and jellies.
• Algin (brown algae) and carrageenan (red algae) are commercially important hydrocolloids.
• Porphyra, Laminaria and Sargassum are among the 70 marine algae used as food.
• Chlorella and Spirulina are unicellular algae rich in proteins, used as food supplements even by space travellers.

Distinctive Features of Bryophyte and Pteridophyte Body

• Bryophytes are called amphibians of the plant kingdom because they live in soil but need water for sexual reproduction.
• In bryophytes the main plant body is the gametophyte (haploid); it is attached by rhizoids and lacks true roots, stem or leaves.
• The bryophyte sporophyte is not free-living; it is attached to the gametophyte and derives nourishment from it.
• Pteridophytes are the first terrestrial plants to have vascular tissues (xylem and phloem).
• In pteridophytes the dominant plant body is the sporophyte (diploid) differentiated into true root, stem and leaves.
• Heterospory (two kinds of spores, macrospores and microspores) is seen in Selaginella and Salvinia, and is a precursor to the seed habit.