Plant Kingdom Essentials

Cheatsheet Content

CHAPTER 3: PLANT KINGDOM Whittaker's Five Kingdom Classification (1969): Monera, Protista, Fungi, Animalia, Plantae. Kingdom Plantae is popularly known as the 'plant kingdom'. Changes over time: Fungi, Monera, and Protista (with cell walls) excluded from Plantae. Cyanobacteria (blue-green algae) are no longer considered 'algae'. This chapter describes Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms under Plantae. Classification Systems Earliest systems: Used gross superficial morphological characters (habit, colour, leaf number/shape). Mainly based on vegetative characters or androecium structure (e.g., Linnaeus). Artificial systems: Separated closely related species; based on few characteristics; gave equal weightage to vegetative and sexual characters (vegetative characters are often affected by environment). Natural classification systems: Based on natural affinities among organisms. Considered external features, and internal features like ultrastructure, anatomy, embryology, and phytochemistry. Given by George Bentham and Joseph Dalton Hooker for flowering plants. Phylogenetic classification systems: Based on evolutionary relationships; acceptable today. Assumes organisms in the same taxa have a common ancestor. Uses information from many sources, especially when fossil evidence is lacking. Modern Taxonomic Tools: Numerical Taxonomy: Easily carried out using computers. Based on all observable characteristics. Numbers and codes assigned to characters, data processed. Each character given equal importance; hundreds of characters considered. Cytotaxonomy: Based on cytological information (chromosome number, structure, behaviour). Chemotaxonomy: Uses chemical constituents of the plant to resolve confusions. 3.1 ALGAE Characteristics: Chlorophyll-bearing, simple, thalloid, autotrophic, largely aquatic (freshwater and marine). Habitats: Moist stones, soils, wood, association with fungi (lichen) and animals (e.g., on sloth bear). Form and Size: Highly variable. Colonial forms: $Volvox$ Filamentous forms: $Ulothrix$, $Spirogyra$ Massive plant bodies (kelps) in marine forms. Reproduction: Vegetative: Fragmentation (each fragment develops into a thallus). Asexual: Production of different types of spores, most common are zoospores. Zoospores: Flagellated (motile), germinate into new plants. Sexual: Fusion of two gametes. Isogamous: Gametes are flagellated and similar in size (e.g., $Ulothrix$) OR non-flagellated (non-motile) but similar in size (e.g., $Spirogyra$). Anisogamous: Gametes dissimilar in size (e.g., species of $Eudorina$). Oogamous: Fusion between one large, non-motile (static) female gamete and a smaller, motile male gamete (e.g., $Volvox$, $Fucus$). Economic Importance: Perform half of Earth's total carbon dioxide fixation via photosynthesis, increasing dissolved oxygen. Primary producers of energy-rich compounds, basis of aquatic food cycles. 70 species of marine algae used as food: $Porphyra$, $Laminaria$, $Sargassum$. Hydrocolloids (water-holding substances) produced by marine brown and red algae: Algin (brown algae) Carrageen (red algae) Agar (from $Gelidium$ and $Gracilaria$): Used to grow microbes and in preparations of ice-creams and jellies. $Chlorella$ (unicellular alga): Rich in proteins, used as food supplement by space travellers. Classes of Algae: Chlorophyceae, Phaeophyceae, Rhodophyceae. 3.1.1 Chlorophyceae (Green Algae) Pigments: Dominance of chlorophyll $a$ and $b$; grass green colour. Plant body: Unicellular, colonial, or filamentous. Chloroplasts: Pigments localized in definite chloroplasts; may be discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped. Storage bodies: One or more pyrenoids located in chloroplasts. Pyrenoids contain protein besides starch. Some store food as oil droplets. Cell wall: Rigid, inner layer of cellulose, outer layer of pectose. Reproduction: Vegetative: Fragmentation. Asexual: Flagellated zoospores produced in zoosporangia. Sexual: Considerable variation; may be isogamous, anisogamous, or oogamous. Examples: $Chlamydomonas$, $Volvox$, $Ulothrix$, $Spirogyra$, $Chara$. 3.1.2 Phaeophyceae (Brown Algae) Habitat: Primarily marine. Size and Form: Great variation. Simple branched, filamentous forms (e.g., $Ectocarpus$). Profusely branched forms (kelps), reaching up to 100 meters. Pigments: Chlorophyll $a$, $c$, carotenoids, xanthophylls. Colour: Olive green to various shades of brown due to amount of xanthophyll pigment, fucoxanthin. Food storage: Complex carbohydrates (laminarin or mannitol). Vegetative cells: Cellulosic wall usually covered by gelatinous coating of algin. Protoplast contains plastids, centrally located vacuole, and nucleus. Plant body structure: Attached to substratum by a holdfast , has a stalk ( stipe ), and leaf-like photosynthetic organ ( frond ). Reproduction: Vegetative: Fragmentation. Asexual: Biflagellate zoospores (pear-shaped, two unequal laterally attached flagella). Sexual: Isogamous, anisogamous, or oogamous. Gamete union in water or within oogonium (oogamous species). Gametes are pyriform (pear-shaped) and bear two laterally attached flagella. Examples: $Ectocarpus$, $Dictyota$, $Laminaria$, $Sargassum$, $Fucus$. 3.1.3 Rhodophyceae (Red Algae) Colour: Red due to predominance of red pigment, r-phycoerythrin. Habitat: Majority marine, greater concentrations in warmer areas. Occur in well-lighted regions close to surface and at great depths with little light penetration. Thalli: Most are multicellular, some have complex body organization. Food storage: Floridean starch, very similar to amylopectin and glycogen. Reproduction: Vegetative: Fragmentation. Asexual: Non-motile spores. Sexual: Oogamous, accompanied by complex post-fertilisation developments. Non-motile gametes. Examples: $Polysiphonia$, $Porphyra$, $Gracilaria$, $Gelidium$. TABLE 3.1 Divisions of Algae and their Main Characteristics Classes Common Name Major Pigments Stored Food Cell Wall Flagellar Number and Position of Insertions Habitat Chlorophyceae Green algae Chlorophyll $a, b$ Starch Cellulose 2-8, equal, apical Fresh water, brackish water, salt water Phaeophyceae Brown algae Chlorophyll $a, c$, fucoxanthin Mannitol, laminarin Cellulose and algin 2, unequal, lateral Fresh water (rare) brackish water, salt water Rhodophyceae Red algae Chlorophyll $a, d$, phycoerythrin Floridean starch Cellulose, pectin and poly sulphate esters Absent Fresh water (some), brackish water, salt water (most) 3.2 BRYOPHYTES Include mosses and liverworts. Found commonly in moist shaded areas in the hills. Amphibians of the plant kingdom: Can live in soil but dependent on water for sexual reproduction. Occur in damp, humid, and shaded localities. Important role in plant succession on bare rocks/soil. Plant body: More differentiated than algae. Thallus-like, prostrate or erect. Attached to substratum by unicellular or multicellular rhizoids. Lack true roots, stem, or leaves, but may possess root-like, leaf-like, or stem-like structures. Main plant body: Haploid, produces gametes, called a gametophyte . Sex organs: Multicellular. Antheridium: Male sex organ, produces biflagellate antherozoids . Archegonium: Female sex organ, flask-shaped, produces a single egg. Fertilization: Antherozoids released into water, contact archegonium, fuse with egg to produce zygote. Zygote does not undergo immediate reduction division. Produces a multicellular body called a sporophyte . Sporophyte is not free-living, attached to photosynthetic gametophyte, derives nourishment from it. Some cells of sporophyte undergo reduction division (meiosis) to produce haploid spores. Spores germinate to produce gametophyte. Economic Importance: Little economic importance generally. Some mosses provide food for herbaceous mammals, birds, and other animals. Species of $Sphagnum$ (a moss): Provides peat (fuel), packing material for trans-shipment of living material (due to water holding capacity). Mosses along with lichens are first organisms to colonize rocks, important ecologically. Decompose rocks, making substrate suitable for higher plants. Form dense mats on soil, reducing impact of rain and preventing soil erosion. Divisions: Liverworts and Mosses. 3.2.1 Liverworts Habitat: Moist, shady habitats (banks of streams, marshy ground, damp soil, bark of trees, deep in woods). Plant body: Thalloid (e.g., $Marchantia$). Thallus is dorsiventral and closely appressed to substratum. Leafy members have tiny leaf-like appendages in two rows on stem-like structures. Asexual reproduction: Fragmentation of thalli. Formation of specialized structures called gemmae (sing. gemma). Gemmae: Green, multicellular, asexual buds. Develop in small receptacles called gemma cups on thalli. Detach from parent body and germinate to form new individuals. Sexual reproduction: Male and female sex organs produced on same or different thalli. Sporophyte differentiated into a foot, seta, and capsule. Spores produced within capsule after meiosis. Spores germinate to form free-living gametophytes. 3.2.2 Mosses Predominant stage: Gametophyte, consists of two stages: Protonema stage: First stage, develops directly from a spore. Creeping, green, branched, frequently filamentous. Leafy stage: Second stage, develops from secondary protonema as a lateral bud. Upright, slender axes with spirally arranged leaves. Attached to soil by multicellular and branched rhizoids. Bears sex organs. Vegetative reproduction: Fragmentation and budding in secondary protonema. Sexual reproduction: Sex organs (antheridia and archegonia) produced at apex of leafy shoots. After fertilization, zygote develops into a sporophyte (foot, seta, and capsule). Sporophyte in mosses is more elaborate than in liverworts. Capsule contains spores. Spores formed after meiosis. Elaborate mechanism of spore dispersal. Common examples: $Funaria$, $Polytrichum$, $Sphagnum$. 3.3 PTERIDOPHYTES Include horsetails and ferns. Uses: Medicinal purposes, soil-binders, ornamentals. Evolutionary significance: First terrestrial plants to possess vascular tissues (xylem and phloem). Habitat: Cool, damp, shady places; some flourish in sandy-soil conditions. Dominant phase: Sporophyte (in bryophytes, it's gametophyte). Main plant body: Sporophyte, differentiated into true root, stem, and leaves. Possess well-differentiated vascular tissues. Leaves: Small (microphylls) as in $Selaginella$. Large (macrophylls) as in ferns. Sporangia: Borne on sporophytes, subtended by leaf-like appendages called sporophylls . Strobili or cones: In some cases, sporophylls form distinct compact structures (e.g., $Selaginella$, $Equisetum$). Sporangia produce spores by meiosis in spore mother cells. Spores germinate to give rise to inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus . Prothallus requirements: Cool, damp, shady places to grow. Limited spread: Due to specific requirements and need for water for fertilization, pteridophytes are limited to narrow geographical regions. Sex organs on gametophytes: Antheridia (male), Archegonia (female). Fertilization: Water required for transfer of antherozoids (male gametes) to archegonium. Fusion of male gamete with egg forms zygote. Zygote produces a multicellular, well-differentiated sporophyte (dominant phase). Homosporous: Majority of pteridophytes produce spores of similar kinds. Heterosporous: Produce two kinds of spores – macro (large) and micro (small) spores (e.g., $Selaginella$, $Salvinia$). Macrospores and microspores germinate to female and male gametophytes, respectively. Female gametophytes retained on parent sporophytes for variable periods. Development of zygotes into young embryos occurs within female gametophytes. This event is a precursor to the seed habit , an important step in evolution. Classification (Four classes): Psilopsida ($Psilotum$) Lycopsida ($Selaginella$, $Lycopodium$) Sphenopsida ($Equisetum$) Pteropsida ($Dryopteris$, $Pteris$, $Adiantum$) 3.4 GYMNOSPERMS Definition: (gymnos: naked, sperma: seeds) Ovules are not enclosed by any ovary wall and remain exposed before and after fertilization. Seeds are naked. Include medium-sized trees, tall trees, and shrubs. Tallest tree species: Giant redwood tree $Sequoia$. Roots: Generally tap roots. Mycorrhiza (fungal association) in some genera (e.g., $Pinus$). Coralloid roots (associated with N$_2$-fixing cyanobacteria) in some others (e.g., $Cycas$). Stems: Unbranched (e.g., $Cycas$). Branched (e.g., $Pinus$, $Cedrus$). Leaves: May be simple or compound. Pinnate leaves in $Cycas$ persist for a few years. Well-adapted to withstand extremes of temperature, humidity, and wind. In conifers, needle-like leaves reduce surface area. Thick cuticle and sunken stomata reduce water loss. Heterosporous: Produce haploid microspores and megaspores. Spores produced within sporangia borne on sporophylls. Sporophylls arranged spirally along an axis to form lax or compact strobili or cones . Male strobili (microsporangiate): Bear microsporophylls and microsporangia. Microspores develop into a highly reduced male gametophytic generation, confined to a limited number of cells, called a pollen grain . Pollen grains develop within microsporangia. Female strobili (macrosporangiate): Bear megasporophylls with ovules or megasporangia. Male and female cones/strobili may be on the same tree (monoecious, e.g., $Pinus$). Male cones and megasporophylls may be on different trees (dioecious, e.g., $Cycas$). Megaspore mother cell differentiates from nucellus cell. Nucellus protected by envelopes, composite structure called an ovule . Ovules borne on megasporophylls (may be clustered to form female cones). Megaspore mother cell divides meiotically to form four megaspores. One megaspore (within megasporangium) develops into a multicellular female gametophyte . Female gametophyte bears two or more archegonia (female sex organs). Female gametophyte retained within megasporangium. Unlike bryophytes and pteridophytes, male and female gametophytes do not have an independent free-living existence; remain within sporangia retained on sporophytes. Pollen grain released from microsporangium, carried by air currents, contacts ovules on megasporophylls. Pollen tube carries male gametes towards archegonia in ovules, discharges contents near mouth of archegonia. Fertilization: Zygote develops into an embryo. Ovules develop into seeds. Seeds are not covered. 3.5 ANGIOSPERMS Flowering plants: Pollen grains and ovules developed in specialised structures called flowers . Seeds are enclosed in fruits. Exceptionally large group of plants, wide range of habitats. Size range: Smallest $Wolffia$ to tall trees of $Eucalyptus$ (over 100 meters). Provide food, fodder, fuel, medicines, and commercial products. Classes: Dicotyledons and Monocotyledons. Dicotyledons vs. Monocotyledons Feature Dicotyledons Monocotyledons Cotyledons Two Single Leaf Venation Reticulate Parallel Floral Whorls Tetramerous or pentamerous (4 or 5 members) Trimerous (3 members) Reproduction in Angiosperms Male Sex Organ: Stamen . Consists of a slender filament with an anther at the tip. Pollen mother cell divides by meiosis to produce microspores, which mature into pollen grains (male gametophyte). Female Sex Organ: Pistil . Consists of a swollen ovary at its base, a long slender style, and stigma. Inside ovary, ovules are present. Each ovule has a megaspore mother cell that undergoes meiosis to form four haploid megaspores. Three megaspores degenerate, one divides to form the embryo sac . Embryo sac: Three-celled egg apparatus (one egg cell and two synergids), three antipodal cells, and two polar nuclei. Polar nuclei eventually fuse to produce a diploid secondary nucleus. Pollination: Pollen grains (after dispersal from anthers) carried by wind or other agencies to the stigma of a pistil. Pollen grains germinate on stigma, pollen tubes grow through tissues of stigma and style to reach ovule. Pollen tubes enter embryo-sac, discharge two male gametes. Double Fertilization (Unique to Angiosperms): One male gamete fuses with the egg cell ( syngamy ) to form a zygote . The other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN) . Post-fertilization developments: Zygote develops into an embryo (with one or two cotyledons). PEN develops into endosperm (provides nourishment to developing embryo). Synergids and antipodals degenerate. Ovules develop into seeds . Ovaries develop into fruit . 3.6 PLANT LIFE CYCLES AND ALTERNATION OF GENERATIONS Plants have both haploid and diploid cells that can divide by mitosis, leading to haploid and diploid plant bodies. Haploid plant body: Produces gametes by mitosis; represents a gametophyte . Diploid sporophytic plant body: Produced by zygote dividing by mitosis. Produces haploid spores by meiosis. Haploid spores divide by mitosis to form haploid plant body. Thus, during the life cycle of sexually reproducing plants, there is an alternation of generations between gamete-producing haploid gametophyte and spore-producing diploid sporophyte. Different plant groups exhibit variations in life cycle patterns: 1. Haplontic Life Cycle Sporophytic generation represented only by the one-celled zygote. No free-living sporophytes. Meiosis in zygote results in haploid spores. Haploid spores divide mitotically to form the gametophyte. Dominant, photosynthetic phase is the free-living gametophyte. Examples: Many algae such as $Volvox$, $Spirogyra$, and some species of $Chlamydomonas$. 2. Diplontic Life Cycle Diploid sporophyte is the dominant, photosynthetic, independent phase of the plant. Gametophytic phase represented by single to few-celled haploid gametophyte. Examples: An alga, $Fucus$ sp. All seed-bearing plants (gymnosperms and angiosperms) follow this pattern with variations (gametophytic phase is few to multi-celled). 3. Haplo-diplontic Life Cycle (Intermediate) Both phases (haploid gametophyte and diploid sporophyte) are multicellular. They differ in their dominant phases. Bryophytes: Dominant, independent, photosynthetic, thalloid or erect haploid gametophyte. Alternates with short-lived multicellular sporophyte (totally or partially dependent on gametophyte for anchorage and nutrition). Pteridophytes: Dominant, independent, photosynthetic, vascular diploid sporophyte. Alternates with multicellular, saprophytic/autotrophic, independent but short-lived haploid gametophyte. Examples: Most algal genera are haplontic. Some algae like $Ectocarpus$, $Polysiphonia$, kelps are haplo-diplontic. $Fucus$ (an alga) is diplontic.