Systems of Classification
Types of Classification Systems
- Practical Classification: Plants classified based on economic importance or human use (food, shelter).
- Artificial Classification: Proposed by Linnaeus AIPMT 1989. Based on one or a few superficial morphological characters (e.g., habit, colour, number/shape of leaves) or specifically Androecium structure AIPMT 1989. Equal weightage is given to vegetative and reproductive characters, which is unacceptable as vegetative characters are easily affected by the environment.
- Natural Classification: Proposed by Bentham and Hooker AIPMT 1988. Based on natural affinities and complete gross morphological characters. It considers internal features like ultrastructure, anatomy, embryology, and phytochemistry. Floral/reproductive characters are given more weightage.
- Phylogenetic Classification: Based on common evolutionary descent and relationships AIPMT 1989, 1994, 2009. Organisms belonging to the same taxa have a common ancestor.
- Phenetic / Numerical Classification: Carried out using computers based on all observable characteristics / overall similarities AIPMT 2003, 2004. Numbers and codes are assigned to all characters; hundreds of characters can be considered simultaneously with equal importance.
Taxonomy and Its Types
Modern Taxonomic Branches:
- Cytotaxonomy: Based on cytological information like chromosome number, structure, and behaviour.
- Chemotaxonomy: Based on the chemical constituents of plants (alkaloids, carotenoids, tannins, nucleic acids).
Algae
General Characteristics & Reproduction
- Chlorophyll-bearing, simple, thalloid, autotrophic aquatic organisms. Plant body is not differentiated into roots, stem, and leaves.
- Occur in symbiotic association with fungi (lichens) and animals (on sloth bears).
- Vegetative reproduction is by fragmentation.
- Asexual reproduction is usually by flagellated zoospores.
- Sexual Reproduction:
- Isogamy: Fusing gametes are morphologically similar. Flagellated: Ulothrix AIPMT 1997, 1998. Non-flagellated: Spirogyra AIPMT 1995 AIPMT 2013.
- Anisogamy: Fusing gametes are dissimilar in size (e.g., Eudorina).
- Oogamy: Fusion between one large, non-motile female gamete and a smaller, motile male gamete AIPMT 2004. (e.g., Volvox, Fucus AIPMT 1991).
- Exception/Crucial detail: Chlamydomonas exhibits complete evolution of sexual reproduction showing isogamy, anisogamy, and oogamy AIPMT 2013.
- Zygotic Meiosis: In Ulothrix and Spirogyra, reduction division (meiosis) occurs at the time of zygospore germination AIPMT 1993. The product of conjugation/fertilization is a zygospore AIPMT 1991.
Economic Importance
- At least half of the total CO2 fixation on earth is carried out by algae. They increase dissolved oxygen levels in the environment AIPMT 2016.
- Primary producers forming the basis of aquatic food cycles.
- Edible Algae: 70 species of marine algae are used as food, including Porphyra, Laminaria, and Sargassum AIPMT 2016.
- Protein-Rich Algae: Unicellular algae like Chlorella and Spirulina NEET 2020, 2021 are rich in protein and used as food supplements by space travellers AIPMT 1997.
- Hydrocolloids (Water-holding substances):
- Algin is obtained from brown algae AIPMT 2016 NEET 2021, 2022.
- Carrageen is obtained from red algae AIPMT 2016 NEET 2021, 2022.
- Agar-agar: Obtained from red algae Gelidium and Gracilaria AIPMT 2016. Used to grow microbes and in ice creams/jellies.
Classes of Algae
| Feature | Chlorophyceae (Green Algae) | Phaeophyceae (Brown Algae) | Rhodophyceae (Red Algae) |
|---|---|---|---|
| Major Pigments | Chlorophyll a, b | Chlorophyll a, c, Fucoxanthin AIPMT 1997 NEET 2022, 2023, 2024 | Chlorophyll a, d, Phycoerythrin AIPMT 2000 NEET 2022, 2023 |
| Stored Food | Starch | Mannitol, Laminarin AIPMT 2009 NEET 2020, 2022, 2024 | Floridean starch (similar to amylopectin and glycogen) NEET 2020 |
| Cell Wall | Cellulose (inner) and Pectose (outer) | Cellulose and Algin | Cellulose, Pectin, and Polysulphate esters |
| Flagella | 2-8, equal, apical | 2, unequal, lateral NEET 2024 | Absent (No motile stage) NEET 2020 |
| Key Examples | Chlamydomonas (cup-shaped chloroplast AIPMT 1993), Volvox (colonial NEET 2017), Ulothrix, Spirogyra, Chara. | Ectocarpus (male gametes flagellated AIPMT 2015), Dictyota, Laminaria, Sargassum, Fucus. | Polysiphonia, Porphyra, Gracilaria, Gelidium. |
- Crucial detail for Green Algae: Storage bodies called pyrenoids are located in chloroplasts. Pyrenoids contain protein besides starch AIPMT 1993, 1995.
- Crucial detail for Brown Algae: Plant body is attached to substratum by a holdfast, has a stalk (stipe), and a leaf-like photosynthetic organ (frond).
Bryophyta
General Characteristics & Life Cycle
- Known as amphibians of the plant kingdom because they live in soil but require water for sexual reproduction (fertilisation) AIPMT 1991, 1992, 2011, 2016.
- Occur in damp, humid, and shaded localities (sciophytes). They are the first land plants.
- Lack true roots, stem, or leaves. Attached to substratum by unicellular or multicellular rhizoids.
- Water conducting tissue is parenchyma (vascular tissues xylem and phloem are absent).
- Main plant body is haploid (Gametophyte). It is independent and dominant.
- Sex Organs: Multicellular and jacketed. Male is antheridium (produces biflagellate antherozoids AIPMT 1998); female is archegonium (flask-shaped, produces a single egg).
- Fertilization: Zoidogamy. Zygote does not undergo immediate meiosis. It forms a multicellular sporophyte.
- Sporophyte: Not free-living; it is attached to and partially/totally parasitic on the photosynthetic gametophyte for nourishment AIPMT 1996, 2005.
Classes of Bryophytes
- Hepaticopsida (Liverworts):
- Thalloid plant body (e.g., Marchantia). Rhizoids are unicellular and unbranched.
- Asexual reproduction by fragmentation or gemmae. Gemmae are green, multicellular, asexual buds in gemma cups NEET 2018, 2021.
- Spore dissemination is aided by hygroscopic elaters.
- Male and female gametophytes are independent and free-living AIPMT 2013.
- Bryopsida (Mosses):
- Gametophyte consists of two stages:
- Protonema stage: Creeping, green, filamentous. Develops directly from a spore NEET 2023.
- Leafy stage: Develops from secondary protonema as a lateral bud.
- Vegetative reproduction by fragmentation and budding.
- Sporophyte (foot, seta, capsule) is more elaborate. Capsule has peristome teeth for spore dispersal.
- Examples: Funaria, Polytrichum, Sphagnum.
- Gametophyte consists of two stages:
- Economic Importance: Sphagnum provides peat (used as fuel) and is highly hygroscopic, used as packing material for trans-shipment of living material AIPMT 2006. Mosses and lichens are pioneer colonizers on bare rocks (ecological succession).
Pteridophyta
General Characteristics & Life Cycle
- Includes whisk ferns, club mosses, horsetails, and ferns. Used for medicinal purposes, soil binders, and ornamentals.
- First terrestrial plants to possess vascular tissues (xylem and phloem) AIPMT 1993 NEET 2012. Xylem lacks vessels; phloem lacks sieve tubes and companion cells.
- Main plant body is a Sporophyte (diploid), differentiated into true roots, stem, and leaves.
- Leaves may be microphylls (small, e.g., Selaginella) or megaphylls (large/fronds, e.g., ferns).
- Sporangia are subtended by leaf-like appendages called sporophylls. In some, they form compact structures called strobili or cones (e.g., Selaginella, Equisetum NEET 2020).
- Gametophyte (Prothallus): Spores germinate to form a small, multicellular, free-living, mostly photosynthetic, and independent haploid prothallus.
- Water is required for the transport of male gametes (antherozoids) to the archegonia AIPMT 2016, restricting their geographical spread.
Heterospory & Seed Habit Precursor
- Most pteridophytes produce similar spores (homosporous).
- Crucial Exception: Genera like Selaginella and Salvinia produce two kinds of spores (macro and microspores) and are heterosporous AIPMT 2008 NEET 2011, 2018, 2021, 2023.
- The female gametophyte in heterosporous plants is retained on the parent sporophyte for variable periods.
- The development of the zygote into a young embryo takes place within the female gametophyte. This event is a precursor to the seed habit, an important evolutionary step AIPMT 1989 NEET 2011, 2019.
Classes of Pteridophyta
- Psilopsida: Most primitive (e.g., Psilotum - living fossil) NEET 2024.
- Lycopsida: Club mosses. Microphyllous (e.g., Selaginella, Lycopodium) NEET 2024.
- Sphenopsida: Horsetails. Stem differentiated into nodes and internodes (e.g., Equisetum) NEET 2024.
- Pteropsida: Ferns. Macrophyllous, leaves show circinate vernation (e.g., Adiantum, Pteris, Dryopteris) NEET 2024.
Gymnosperms
General Characteristics & Life Cycle
- Plants in which the ovules are not enclosed by any ovary wall and remain exposed (naked seeds) both before and after fertilization NEET 2018, 2020.
- All gymnosperms are heterosporous NEET 2020. They produce haploid microspores and megaspores.
- Chief water-conducting elements are tracheids (vessels are absent).
- Unlike bryophytes and pteridophytes, the male and female gametophytes do not have an independent free-living existence NEET 2020. They remain within sporangia retained on the sporophyte.
- Pollen grains are released from microsporangia and carried by air currents (wind/anemophily) directly to the mouth of the archegonia on the megasporophylls NEET 2022, 2023.
- Endosperm is haploid (n) and forms before fertilization.
Key Examples & Features
- Roots: Generally tap roots.
- Pinus has an obligate fungal association called mycorrhiza (seeds cannot germinate without it NEET 2019).
- Cycas has specialized coralloid roots associated with N2-fixing cyanobacteria NEET 2022.
- Stem: Unbranched in Cycas NEET 2018; branched in Pinus and Cedrus.
- Leaves: Conifers have needle-like leaves, thick cuticles, and sunken stomata to tolerate extreme environmental conditions AIPMT 2016 NEET 2020.
- Plant Sex: Pinus is monoecious (male and female cones on same plant); Cycas is dioecious (male cones and megasporophylls on different trees) NEET 2017. Pinus pollen grains are winged NEET 2018.
- Sequoia (giant redwood) is one of the tallest tree species AIPMT 2016.
Angiosperms
General Characteristics & Double Fertilization
- Seeds are enclosed by fruits (ovary develops into fruit, ovules into seeds).
- Microscopic Wolffia is the smallest; Eucalyptus is the tallest.
- Divided into Dicotyledons (two cotyledons, reticulate venation, tetramerous/pentamerous) and Monocotyledons (one cotyledon, parallel venation, trimerous).
- Embryo Sac: 7-celled, 8-nucleated female gametophyte. Contains a 3-celled egg apparatus (1 egg + 2 synergids), 3 antipodal cells, and a large central cell with 2 polar nuclei.
- Double Fertilization: A unique event to angiosperms involving two fusions:
- Syngamy: One male gamete fuses with the egg cell to form a diploid zygote (2n).
- Triple Fusion: The other male gamete fuses with the diploid secondary nucleus (formed by fusion of two polar nuclei) to form a triploid Primary Endosperm Nucleus (PEN; 3n).