PLANT KINGDOM:
Chapter at a glance
CLASSIFICATION SYSTEMS
1. Artificial classification systems
Based on vegetative characters or morphological characters. E.g. Linnaeus’s artificial system (based on androecium structure).
Drawbacks: It separates closely related species. Equal weightage to vegetative and sexual characters.
2. Natural classification systems
Based on natural affinities among organisms. Considers external and internal features. E.g. Classification of flowering plants by Bentham & Hooker.
3. Phylogenetic classification systems
Based on evolutionary relationships.
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Numerical Taxonomy: Based on all observable characters. It is easily carried out using computers.
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Cytotaxonomy: Based on cytological information.
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Chemotaxonomy: It uses chemical constituents of plants.
ALGAE
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Simple, thalloid, autotrophic, chlorophyll-bearing.
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Some are associated with fungi (lichen) and animals (e.g. sloth bear).
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Unicellular forms: E.g. Chlamydomonas.
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Colonial forms: E.g. Volvox.
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Filamentous forms: E.g. Ulothrix and Spirogyra.
Reproduction:
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Vegetative: By fragmentation.
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Asexual: By spores. E.g. zoospores (most common).
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Sexual: Fusion of two gametes. Many types:
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Isogamous: Fusion of similar gametes. They may be flagellated (Ulothrix) or non-flagellated (Spirogyra).
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Anisogamous: Fusion of dissimilar gametes. E.g. Eudorina.
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Oogamous: Large, non-motile female gamete + small, motile male gamete. E.g. Volvox, Fucus.
Benefits of algae:
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Fix half of the total CO2 through photosynthesis.
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Basis of the food cycles of aquatic animals.
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Used as food. E.g. Porphyra, Laminaria and Sargassum.
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Agar (from Gelidium & Gracilaria) is used to grow microbes and in ice-creams and jellies.
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Some produce hydrocolloids (water holding substances). E.g. algin (brown algae) and carrageen (red algae).
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Chlorella & Spirulina: Food for space travelers.
| Algae Classes | Chlorophyceae (Green algae) | Phaeophyceae (brown algae) | Rhodophyceae (Red algae) |
|---|---|---|---|
| Major pigments | Chlorophyll a, b | Chlorophyll a, c, Fucoxanthin | Chlorophyll a, d, r-phycoerythrin |
| Stored food | Starch. Most have pyrenoids (storage bodies) in chloroplasts. | Mannitol, laminarin | Floridean Starch |
| Cell wall | Inner cellulose + outer pectose. | Cellulose and algin | Cellulose |
| Flagellar number & position. | 2-8, equal, apical | 2, unequal, lateral | Absent |
| Habitat | Fresh, salt & brackish waters | Fresh (rare), salt & brackish waters | Fresh, salt (most) & brackish waters |
| Examples | Chlamydomonas, Volvox, Ulothrix, Spirogyra & Chara. | Ectocarpus, Dictyota, Laminaria, Sargassum & Fucus. | Polysiphonia, Porphyra, Gracilaria and Gelidium |
BRYOPHYTES (Liverworts & Mosses)
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Amphibians of the plant kingdom.
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Thallus-like body. Attached to substratum by rhizoids.
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No true roots, stem or leaves.
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Main plant body is haploid (gametophyte) → gametes.
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Male sex organ (antheridium) produces biflagellate antherozoids. Female sex organ (archegonium) produces an egg.
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Antherozoids → to water → archegonium → antherozoid + egg → zygote → sporophyte → attached to gametophyte → Some sporophyte cells undergo meiosis → haploid spores → germinate → gametophyte.
Importance of Bryophytes:
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Food for mammals, birds etc.
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Peat from Sphagnum is used as fuel and packing material.
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Role in plant succession.
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Prevent soil erosion.
Liverworts
Thalloid plant body. E.g. Marchantia.
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Asexual reproduction: By fragmentation or by formation of gemmae (asexual buds).
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Gemmae develop in small receptacles (gemma cups). Gemmae germinate to new individuals.
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Sexual reproduction: Sporophyte is differentiated into a foot, seta & capsule. After meiosis, spores are formed in capsule → gametophyte.
Mosses
Gametophyte (predominant stage) consists of 2 stages:
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Protonema stage: Develops from a spore. Green, branched and filamentous.
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Leafy stage: Develops from secondary protonema. Spirally arranged leaves.
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Vegetative reproduction: By fragmentation & budding.
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Sexual reproduction: Antheridia & archegonia are produced. Zygote develops into a sporophyte. Spores are formed in capsule after meiosis. E.g. Funaria, Polytrichum and Sphagnum.
PTERIDOPHYTES
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Include horsetails and ferns.
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First terrestrial plants to possess vascular tissues.
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Dominant phase is a sporophyte. It is differentiated to true root, stem & leaves.
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Leaves are small (microphylls) as in Selaginella or large (macrophylls) as in ferns.
REPRODUCTION:
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Sporophytes bear sporangia & sporophylls. In some cases, sporophylls form strobili or cones (E.g. Selaginella, Equisetum).
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Sporangia → spore mother cells → meiosis → spores → prothallus (free-living thalloid gametophytes).
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Prothallus needs water for fertilization. So, the spread of pteridophytes is limited to narrow geographical regions.
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Prothallus bears antheridia and archegonia.
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Antherozoids (male gametes from antheridia) → via water → to archegonium → fuses with egg in archegonium → zygote → sporophyte.
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Most pteridophytes are homosporous (produce similar kinds of spores). Others are heterosporous (2 kinds of spores: macro & micro). E.g. Selaginella & Salvinia.
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Megaspores & microspores germinate to form female and male gametophytes, respectively.
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In female gametophytes, zygotes develop to embryos.
Pteridophytes have 4 classes:
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Pslopsida: E.g. Psilotum
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Lycopsida: E.g. Selaginella, Lycopodium
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Sphenopsida: E.g. Equisetum
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Pteropsida: E.g. Dryopteris, Pteris, Adiantum
GYMNOSPERMS
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Ovules are not enclosed by ovary wall. Seeds are naked.
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Sequoia (giant redwood) is the tallest tree species.
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Generally tap roots. Roots in some genera have fungal association (mycorrhiza. E.g. Pinus).
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Roots of Cycas etc. are associated with N2-fixing cyanobacteria (coralloid roots).
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Stems are unbranched (Cycas) or branched (Pinus, Cedrus).
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In conifers (Pinus, Cedrus etc.), needle-like leaves reduce the surface area. Their thick cuticle and sunken stomata reduce water loss.
REPRODUCTION:
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Heterosporous- haploid microspores and megaspores.
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Some leaves become sporophylls. They bear sporangia.
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Sporophylls are 2 types:
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Microsporophylls: Arranged to male strobili (microsporangiate). They bear microsporangia → microspores → male gametophytes → pollen grains.
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Megasporophylls: Arranged to female strobili (macrosporangiate). They bear megasporangia (ovules). Megasporangium consists of nucellus. A cell of nucellus differentiates to megaspore mother cell → meiosis → 4 megaspores. One megaspore develops to female gametophyte that bears archegonia.
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Male or female cones may be on same tree (Pinus) or on different trees (Cycas).
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Pollen grain from microsporangium → via air → to opening of ovules → male gametes move through pollen tube → archegonia → fertilization → zygote → embryo.
ANGIOSPERMS (FLOWERING PLANTS)
2 classes:
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Dicotyledons: 2 cotyledons in seeds, reticulate venation.
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Monocotyledons: One cotyledon, parallel venation.
REPRODUCTION:
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Flower is the reproductive structure.
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Male sex organ: stamen (filament + anther).
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Pollen mother cell in anthers → meiosis → microspores → pollen grains.
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Female sex organ: Pistil (ovary + style + stigma).
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Ovary contains ovules. An ovule has a megaspore mother cell → meiosis → 4 megaspores (n) → 3 of them degenerate and one forms embryo sac.
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Each embryo sac has a 3-celled egg apparatus (1 egg cell & 2 synergids), 3 antipodal cells & 2 polar nuclei. The polar nuclei fuse to produce a secondary nucleus (2n).
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Pollen grains → to stigma (pollination) → germinate on stigma → pollen tubes form → reach the ovule → enter the embryo-sac → 2 male gametes are discharged.
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One male gamete + egg cell → zygote → Embryo.
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Other male gamete + secondary nucleus (2n) → triploid primary endosperm nucleus (PEN) → endosperm (nourishes embryo).
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Thus there are two fusions. So it is called double fertilisation. It is an event unique to angiosperms.
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Synergids & antipodals degenerate after fertilization.
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Ovules develop to seeds and ovaries develop to fruit.
PLANT LIFE CYCLES AND ALTERNATION OF GENERATIONS
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Angiosperms show alternation of generations between gametophyte (haploid) and sporophyte (diploid).
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Gametophyte (n) → mitosis → gametes → fertilization → zygote → mitosis → sporophyte (2n) → meiosis → haploid spores → mitosis → Gametophyte.
Patterns of Plant life cycles
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Haplontic: Sporophyte is represented only by zygote. Zygote → meiosis → spores (n) → mitosis soup → gametophyte (dominant, photosynthetic). E.g. Algae such as Volvox, Spirogyra and Chlamydomonas.
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Diplontic: Sporophyte is dominant & photosynthetic. E.g. An alga, Fucus sp., gymnosperms & angiosperms.
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Haplo-diplontic: Both gametophyte & sporophyte are multicellular. But they have different dominant phases. E.g. Bryophytes & Pteridophytes.
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