Class 11 Biology Notes Chapter 3 (Plant kingdom) – Biology Book
Alright students, let's begin our detailed study of Chapter 3: Plant Kingdom. This is a crucial chapter for understanding the diversity and evolution of plants, and frequently tested in government exams. Pay close attention to the distinguishing features of each group and the life cycle patterns.
Chapter 3: Plant Kingdom - Detailed Notes
1. Introduction & Basis of Classification
- Plant Kingdom: Includes eukaryotic, multicellular, autotrophic organisms, primarily characterized by the presence of chlorophyll and cell walls (mainly cellulose).
- Evolution of Classification Systems:
- Artificial System (e.g., Linnaeus): Based on superficial morphological characters (like habit, number/structure of stamens). Easy to use but separated closely related species and grouped unrelated ones. Gave equal weightage to vegetative and sexual characteristics (vegetative characters are more easily affected by environment).
- Natural System (e.g., Bentham & Hooker): Based on natural affinities among organisms, considering both external and internal features (ultrastructure, anatomy, embryology, phytochemistry). Better reflects natural relationships.
- Phylogenetic System (e.g., Engler & Prantl, Hutchinson): Based on evolutionary relationships between organisms. Assumes organisms belonging to the same taxa have a common ancestor. Uses fossil evidence and is currently the most accepted system.
- Numerical Taxonomy: Uses computers, based on all observable characteristics. Number and codes assigned to characters, data processed. Each character gets equal importance.
- Cytotaxonomy: Based on cytological information like chromosome number, structure, and behaviour.
- Chemotaxonomy: Uses chemical constituents of plants (e.g., specific proteins, DNA sequences, secondary metabolites) to resolve taxonomic confusion.
2. Algae
- General Characteristics:
- Chlorophyll-bearing, simple, thalloid (undifferentiated body), autotrophic, largely aquatic (freshwater & marine).
- Occur in moist stones, soils, wood, sometimes in association with fungi (lichens) and animals (e.g., sloth bear).
- Form & Size: Highly variable - microscopic unicellular (Chlamydomonas), colonial (Volvox), filamentous (Ulothrix, Spirogyra), massive plant bodies (Kelp).
- Reproduction:
- Vegetative: Fragmentation.
- Asexual: Production of spores, most common being zoospores (flagellated, motile).
- Sexual: Fusion of two gametes.
- Isogamous: Fusion of gametes similar in size (flagellated - Chlamydomonas / non-flagellated - Spirogyra).
- Anisogamous: Fusion of two gametes dissimilar in size (Some species of Chlamydomonas).
- Oogamous: Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete (Volvox, Fucus).
- Economic Importance:
- At least half of total CO2 fixation on Earth. Increase dissolved O2.
- Primary producers in aquatic ecosystems.
- Food source: Porphyra, Laminaria, Sargassum.
- Agar: Commercial product from Gelidium and Gracilaria (used in microbiology media, ice-creams, jellies).
- Algin: Hydrocolloid (water holding substance) from brown algae.
- Carrageenan: Hydrocolloid from red algae.
- Chlorella & Spirullina: Unicellular, rich in protein, used as food supplements (even by space travellers).
- Classes of Algae:
- Chlorophyceae (Green Algae):
- Habitat: Mostly freshwater, some marine/brackish.
- Pigments: Chlorophyll a and b (dominant, giving grass green colour). Located in chloroplasts (various shapes: discoid, plate-like, reticulate, cup-shaped, spiral/ribbon-shaped).
- Storage: Starch. Many store food as pyrenoids (storage bodies located in chloroplasts, contain protein besides starch). Some store food as oil droplets.
- Cell Wall: Inner layer of cellulose, outer layer of pectose.
- Reproduction: Vegetative (fragmentation, different types of spores), Asexual (flagellated zoospores in zoosporangia), Sexual (iso-, aniso-, oogamous).
- Flagella: 2-8, equal, apical.
- Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.
- Phaeophyceae (Brown Algae):
- Habitat: Primarily marine.
- Size: Simple branched, filamentous (Ectocarpus) to profusely branched forms like kelps (up to 100m).
- Pigments: Chlorophyll a, c, carotenoids, fucoxanthin (xanthophyll, gives olive green to brown colour).
- Storage: Complex carbohydrates - Laminarin or Mannitol.
- Cell Wall: Cellulose, usually covered by a gelatinous coating of algin.
- Body Structure: Attached to substratum by holdfast, has a stalk (stipe), and leaf-like photosynthetic organ (frond). Possess air bladders in some.
- Reproduction: Vegetative (fragmentation), Asexual (biflagellate zoospores - pear-shaped, two unequal laterally attached flagella), Sexual (iso-, aniso-, oogamous; gametes pyriform, bear two lateral flagella).
- Examples: Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.
- Rhodophyceae (Red Algae):
- Habitat: Mostly marine, greater concentrations in warmer areas. Found in well-lighted regions close to surface and at great depths where little light penetrates.
- Pigments: Chlorophyll a, d, r-phycoerythrin (red pigment, dominant, allows absorption of blue-green light).
- Body: Multicellular, complex body organisation in most.
- Storage: Floridean Starch (structurally similar to amylopectin and glycogen).
- Cell Wall: Cellulose, pectin, and polysulphate esters.
- Reproduction: Vegetative (fragmentation), Asexual (non-motile spores), Sexual (oogamous, non-motile gametes). Complex post-fertilisation developments.
- Examples: Polysiphonia, Porphyra, Gracilaria, Gelidium.
- Chlorophyceae (Green Algae):
3. Bryophytes
- General Characteristics:
- Includes mosses, liverworts, hornworts.
- Habitat: Moist, shaded areas in hills.
- Called "Amphibians of the Plant Kingdom" because they live in soil but depend on water for sexual reproduction.
- Play important role in plant succession on bare rocks/soil.
- Body: Thallus-like, prostrate or erect, attached by unicellular or multicellular rhizoids. Lack true roots, stem, leaves (may have root-like, leaf-like, stem-like structures).
- Main plant body is haploid (n) - Gametophyte. Produces gametes.
- Sex Organs: Multicellular. Male - Antheridium (produces biflagellate antherozoids). Female - Archegonium (flask-shaped, produces single egg).
- Fertilization: Antherozoids released into water, fuse with egg in archegonium to form zygote (2n).
- Sporophyte (2n): Zygote develops into a multicellular sporophyte. It is not free-living but attached to the photosynthetic gametophyte (dependent on it for nourishment). Some cells undergo meiosis to produce haploid spores (n).
- Germination: Spores germinate to produce gametophyte.
- Economic Importance:
- Food for herbaceous mammals, birds.
- Sphagnum (a moss) provides peat (fuel, packing material for trans-shipment of living material due to water holding capacity).
- Ecological Importance: Decompose rocks (along with lichens), making substrate suitable for higher plants (pioneers of succession). Form dense mats on soil, reduce soil erosion.
- Classes:
- Liverworts:
- Habitat: Moist, shady habitats (banks of streams, marshy ground, damp soil, bark, deep woods).
- Body: Thalloid (Marchantia). Thallus dorsiventral.
- Asexual Reproduction: Fragmentation, or formation of specialised structures called gemmae (green, multicellular buds developed in gemma cups on thalli; detach and form new individuals).
- Sexual Reproduction: Male and female sex organs on same or different thalli.
- Sporophyte: Differentiated into foot, seta, capsule. Spores produced within capsule after meiosis.
- Mosses:
- Predominant stage is gametophyte, consisting of two stages:
- Protonema stage: Develops directly from spore. Creeping, green, branched, filamentous stage.
- Leafy stage: Develops from secondary protonema as a lateral bud. Upright, slender axes bearing spirally arranged leaves. Attached by multicellular, branched rhizoids. Bears sex organs.
- Vegetative Reproduction: Fragmentation and budding in the secondary protonema.
- Sexual Reproduction: Antheridia and archegonia at apex of leafy shoots. Zygote develops into sporophyte (foot, seta, capsule).
- Sporophyte: More elaborate than liverworts. Contains spores. Has an elaborate mechanism of spore dispersal.
- Examples: Funaria, Polytrichum, Sphagnum.
- Predominant stage is gametophyte, consisting of two stages:
- Liverworts:
4. Pteridophytes
- General Characteristics:
- Includes horsetails and ferns.
- Habitat: Cool, damp, shady places (some flourish well in sandy-soil).
- Evolutionary Significance: First terrestrial plants to possess vascular tissues (xylem and phloem).
- Main plant body is diploid (2n) - Sporophyte. Differentiated into true root, stem, and leaves.
- Leaves: Small (microphylls) as in Selaginella or large (macrophylls) as in ferns.
- Sporophylls: Leaves bearing sporangia. May form distinct compact structures called strobili or cones (Selaginella, Equisetum).
- Sporangia: Produce spores by meiosis in spore mother cells.
- Spores germinate into inconspicuous, small, multicellular, free-living, mostly photosynthetic thalloid gametophyte called prothallus (n).
- Prothallus requires cool, damp, shady places to grow. Bears antheridia and archegonia.
- Fertilization: Requires water for transfer of antherozoids (flagellated) to archegonium. Zygote develops into multicellular, well-differentiated sporophyte.
- Spore Types:
- Homosporous: Produce spores of similar kinds (most pteridophytes).
- Heterosporous: Produce two kinds of spores - macro (mega) and micro spores (Selaginella, Salvinia). Megaspores germinate into female gametophytes; microspores into male gametophytes.
- Seed Habit: Female gametophytes retained on parent sporophyte for variable periods. Development of zygote into young embryo takes place within female gametophyte. This event is a precursor to the seed habit, considered an important step in evolution.
- Economic Importance: Medicinal purposes, soil-binders, ornamentals.
- Classes: Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum), Pteropsida (Dryopteris, Pteris, Adiantum).
5. Gymnosperms
- General Characteristics:
- Plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilization. Seeds that develop post-fertilization are naked.
- Includes medium-sized trees, tall trees, shrubs. Giant redwood Sequoia is one of the tallest trees.
- Roots: Generally tap roots.
- Mycorrhiza: Fungal association in roots (Pinus).
- Coralloid Roots: Association with N2-fixing cyanobacteria in Cycas.
- Stems: Unbranched (Cycas) or branched (Pinus, Cedrus).
- Leaves: Simple or compound. Well-adapted to withstand extremes of temperature, humidity, wind. Needle-like leaves with thick cuticle and sunken stomata reduce water loss in conifers.
- Reproduction:
- Heterosporous: Produce haploid microspores and megaspores.
- Spores produced within sporangia borne on sporophylls, arranged spirally along an axis to form compact strobili or cones.
- Male Cones (Microsporangiate): Microsporophylls bear microsporangia. Microspores develop into male gametophyte (highly reduced, confined to limited number of cells = pollen grain).
- Female Cones (Megasporangiate): Megasporophylls bear megasporangia (ovules). Ovule contains megaspore mother cell -> meiosis -> four megaspores -> one develops into multicellular female gametophyte bearing archegonia.
- Pollination: Pollen grains carried by air currents (anemophily) reach ovules.
- Fertilization: Pollen tube carrying male gametes grows towards archegonia in ovules, discharges contents near mouth of archegonia. Fusion of male gamete with egg -> zygote -> embryo. Ovules develop into seeds.
- Endosperm: Female gametophyte provides nutrition; it is haploid (n) and forms before fertilization.
- Examples: Cycas, Pinus, Cedrus, Ginkgo, Sequoia, Ephedra.
6. Angiosperms (Flowering Plants)
- General Characteristics:
- Pollen grains and ovules develop in specialized structures called flowers.
- Seeds are enclosed within fruits (ovules enclosed within ovary).
- Exceptionally large group, wide range of habitats. Size ranges from tiny Wolffia to tall Eucalyptus.
- Provide food, fodder, fuel, medicines, etc.
- Divided into two classes: Dicotyledons (two cotyledons in seed, reticulate venation, tap root, tetra/pentamerous flowers) and Monocotyledons (one cotyledon, parallel venation, fibrous root, trimerous flowers).
- Reproduction:
- Flower: Reproductive unit.
- Stamen: Male sex organ (filament + anther). Anther produces pollen grains after meiosis.
- Pistil/Carpel: Female sex organ (stigma + style + ovary). Ovary contains ovules.
- Ovule: Contains female gametophyte (embryo sac), which has a 3-celled egg apparatus (one egg cell, two synergids), 3 antipodal cells, and 2 polar nuclei.
- Pollination: Transfer of pollen from anther to stigma (by wind, water, insects, animals).
- Fertilization: Pollen germinates on stigma, pollen tube grows through style to reach ovule, enters embryo sac. Discharges two male gametes.
- Syngamy: One male gamete fuses with egg cell -> Zygote (2n).
- Triple Fusion: Other male gamete fuses with diploid secondary nucleus (formed by fusion of 2 polar nuclei) -> Primary Endosperm Nucleus (PEN) (3n).
- Double Fertilization: Occurrence of two fusion events (syngamy + triple fusion). Unique to angiosperms.
- Post-fertilization: Zygote -> Embryo. PEN -> Endosperm (triploid, provides nourishment). Ovules -> Seeds. Ovary -> Fruit.
- Examples: Wheat, Rice, Maize, Mango, Rose, Pea, Sunflower, etc.
7. Plant Life Cycles and Alternation of Generations
- Both haploid (n) and diploid (2n) cells can divide by mitosis in plants, leading to haploid and diploid plant bodies.
- Haploid plant body (Gametophyte) produces gametes by mitosis.
- Diploid plant body (Sporophyte) produces haploid spores by meiosis.
- Alternation of Generations: Life cycle involves alternation between gamete-producing haploid gametophyte and spore-producing diploid sporophyte.
- Life Cycle Patterns:
- Haplontic: Dominant phase is photosynthetic, free-living gametophyte (n). Sporophyte represented only by the zygote (2n). Zygote undergoes meiosis to form haploid spores -> gametophyte. (e.g., Volvox, Spirogyra, some Chlamydomonas).
- Diplontic: Dominant phase is photosynthetic, independent sporophyte (2n). Gametophyte phase represented by single to few-celled haploid gametophyte (pollen grain, embryo sac). Gametic meiosis (meiosis forms gametes). (e.g., All seed-bearing plants - Gymnosperms & Angiosperms; Fucus - an alga).
- Haplo-diplontic: Both sporophyte (2n) and gametophyte (n) phases are multicellular and often free-living, but have different dominant phases.
- Bryophytes: Dominant, independent, photosynthetic gametophyte; short-lived, dependent sporophyte.
- Pteridophytes: Dominant, independent, photosynthetic sporophyte; short-lived, independent (mostly) gametophyte.
- (Also seen in some algae like Ectocarpus, Polysiphonia, Kelps).
Multiple Choice Questions (MCQs)
-
Which classification system is based on evolutionary relationships between various organisms?
a) Artificial system
b) Natural system
c) Phylogenetic system
d) Numerical taxonomy -
Floridean starch as reserve food material and absence of flagellated stages are characteristic features of:
a) Chlorophyceae
b) Phaeophyceae
c) Rhodophyceae
d) Cyanobacteria -
In Bryophytes, the dominant phase of the life cycle is the _______ and the _______ is dependent on it.
a) Sporophyte, Gametophyte
b) Gametophyte, Sporophyte
c) Gametophyte, Zygote
d) Sporophyte, Spore -
Which of the following Pteridophytes is heterosporous?
a) Dryopteris
b) Equisetum
c) Adiantum
d) Selaginella -
Gemmae are specialized structures for asexual reproduction found in:
a) Mosses like Funaria
b) Ferns like Dryopteris
c) Liverworts like Marchantia
d) Gymnosperms like Cycas -
In Gymnosperms, the endosperm is:
a) Diploid (2n) and formed before fertilization
b) Haploid (n) and formed before fertilization
c) Triploid (3n) and formed after fertilization
d) Haploid (n) and formed after fertilization -
Double fertilization is a unique characteristic feature of:
a) Algae
b) Bryophytes
c) Gymnosperms
d) Angiosperms -
The main plant body is differentiated into true root, stem, and leaves in which group?
a) Algae
b) Bryophytes
c) Pteridophytes
d) Fungi -
Which pigment is responsible for the characteristic brown colour of Phaeophyceae?
a) Chlorophyll b
b) Fucoxanthin
c) Phycoerythrin
d) Phycocyanin -
A plant shows thallus level organization, lacks true roots, stem and leaves, possesses rhizoids, and its main plant body is haploid. It needs water for fertilization. This plant likely belongs to:
a) Pteridophytes
b) Gymnosperms
c) Angiosperms
d) Bryophytes
Answer Key for MCQs:
- (c)
- (c)
- (b)
- (d)
- (c)
- (b)
- (d)
- (c)
- (b)
- (d)
Revise these notes thoroughly, focusing on the comparative features and life cycle patterns. Understanding the progression from simpler algae to complex angiosperms is key. Good luck with your preparation!