Plant Kingdom

Key Concepts

Classification Systems in Plants

• Artificial Systems: Based on superficial morphological characters like habit, color, and leaf shape. Linnaeus's system based on androecium structure is an example. Drawback: Gave equal weightage to vegetative and sexual characters.
• Natural Systems: Based on natural affinities and consider both external and internal features like anatomy, embryology, and phytochemistry. Proposed by Bentham and Hooker.
• Phylogenetic Systems: Based on evolutionary relationships. Assumes organisms in the same taxa have a common ancestor.
• Modern Taxonomic Tools:
  • Numerical Taxonomy: Uses computers to process hundreds of observable characteristics.
  • Cytotaxonomy: Based on cytological information like chromosome number and structure.
  • Chemotaxonomy: Uses chemical constituents of plants.

Algae

Chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic organisms.

• Reproduction: Vegetative (fragmentation), Asexual (zoospores), and Sexual (isogamous, anisogamous, or oogamous).
• Classification of Algae:
  1. Chlorophyceae (Green Algae): Pigments chlorophyll a and b. Store food as pyrenoids (protein + starch). Cell wall: inner cellulose, outer pectose. (e.g., Chlamydomonas, Volvox, Spirogyra).
  2. Phaeophyceae (Brown Algae): Pigments chlorophyll a, c, and fucoxanthin. Store food as laminarin or mannitol. Cell wall has algin coating. (e.g., Ectocarpus, Laminaria, Fucus).
  3. Rhodophyceae (Red Algae): Pigment r-phycoerythrin. Store food as floridean starch. Non-motile gametes and spores. (e.g., Polysiphonia, Porphyra, Gelidium).
• Economic Importance: Carbon dioxide fixation, primary producers, food source (Porphyra), hydrocolloids (algin, carrageen), agar production, and food supplements (Chlorella).

Bryophytes (Amphibians of the Plant Kingdom)

Live in soil but depend on water for sexual reproduction. Found in damp, shaded areas.

• Structure: Thallus-like, attached by rhizoids. Lack true roots, stems, or leaves.
• Life Cycle: Gametophyte (haploid) is the dominant phase. Produces antherozoids (male) in antheridia and eggs (female) in archegonia. Zygote develops into a sporophyte which is dependent on the gametophyte.
• Groups:
  • Liverworts: Thalloid body (e.g., Marchantia). Asexual reproduction by gemmae in gemma cups.
  • Mosses: Life cycle has two stages: protonema and leafy stage. Attached by multicellular rhizoids. (e.g., Funaria, Sphagnum).
• Ecological Importance: Colonize rocks with lichens, prevent soil erosion, and provide peat (Sphagnum).

Pteridophytes

First terrestrial plants to possess vascular tissues (xylem and phloem).

• Structure: Main plant body is a sporophyte (diploid) with true roots, stems, and leaves. Leaves can be small (microphylls, e.g., Selaginella) or large (macrophylls, e.g., Ferns).
• Reproduction: Sporophylls bear sporangia which produce spores by meiosis. Spores germinate into a small, photosynthetic gametophyte called prothallus.
• Heterospory: Production of two kinds of spores (macro and micro). Precursor to the seed habit. (e.g., Selaginella, Salvinia).
• Classes: Psilopsida, Lycopsida, Sphenopsida, Pteropsida.

Gymnosperms (Naked-Seeded Plants)

Ovules are not enclosed by an ovary wall and remain exposed.

• Features: Include tall trees like Sequoia (Giant Redwood). Roots may have fungal association (mycorrhiza in Pinus) or N2-fixing cyanobacteria (coralloid roots in Cycas).
• Life Cycle: Heterosporous (microspores and megaspores). Spores produced in strobili or cones. Pollen grains (reduced male gametophyte) are carried by wind. Seeds are not covered by a fruit.

Angiosperms (Flowering Plants)

Seeds are enclosed in fruits. Pollen and ovules develop in flowers.

• Classes:
  1. Dicotyledons: Seeds with two cotyledons, reticulate venation, tetramerous/pentamerous flowers.
  2. Monocotyledons: Seeds with one cotyledon, parallel venation, trimerous flowers.
• Range from tiny Wolffia to massive Eucalyptus.