Plant Kingdom - Notes | Class 11 | Part 1: Classification systems, Algae

Plant Kingdom Notes - Classification Systems and Algae

Systems of Biological Classification

Earliest systems: Artificial Classification Systems.
Based on vegetative characters or superficial morphological characters like habit, colour, number, and shape of leaves.
Linnaeus’s artificial system based on androecium structure.
Drawbacks:
- Separated closely related species due to reliance on few characteristics.
- Equal weightage to vegetative and sexual characteristics, which is not ideal as vegetative characters are more environment-dependent.
Natural Classification Systems: Based on natural affinities among organisms.
Consider external and internal features (ultrastructure, anatomy, embryology, phytochemistry).
E.g., Classification for flowering plants by George Bentham & Joseph Dalton Hooker.
Phylogenetic Classification Systems: Based on evolutionary relationships, assuming organisms in the same taxa share a common ancestor.
Other sources to resolve classification problems:
- Numerical Taxonomy: Uses all observable characteristics, computer-processed with assigned numbers and codes for equal consideration of hundreds of characters.
- Cytotaxonomy: Based on cytological information like chromosome number, structure, behaviour.
- Chemotaxonomy: Uses chemical constituents of plants.

Simple, thalloid, autotrophic, chlorophyll-bearing, and aquatic (fresh water & marine) organisms.
Also occur in moist stones, soils, and wood.
Some in association with fungi (lichens) and animals (e.g., on sloth bear).
Highly variable form and size:
- Microscopic unicellular: E.g., Chlamydomonas.
- Colonial: E.g., Volvox.
- Filamentous: E.g., Ulothrix, Spirogyra.

Vegetative reproduction: By fragmentation, each fragment develops into a thallus.
Asexual reproduction: By spores, e.g., zoospores (most common, flagellated, motile, germinate into new plants).
Sexual reproduction: Fusion of two gametes, of types:
- Isogamous: Fusion of similar-sized gametes, flagellated (e.g., Ulothrix) or non-flagellated (e.g., Spirogyra).
- Anisogamous: Fusion of dissimilar-sized gametes, e.g., some Eudorina species.
- Oogamous: Fusion of large, non-motile female gamete and smaller, motile male gamete, e.g., Volvox, Fucus.

Fix half of Earth’s CO₂ through photosynthesis, increasing dissolved oxygen levels.
Primary producers, forming the basis of aquatic food cycles.
70 marine algae species used as food, e.g., Porphyra, Laminaria, Sargassum.
Agar (from Gelidium, Gracilaria) used for microbial growth, ice creams, and jellies.
Marine brown & red algae produce hydrocolloids (water-holding substances), e.g., algin (brown algae), carrageen (red algae), used commercially.
Protein-rich unicellular algae like Chlorella, Spirulina used as food supplements by space travellers.

Algae include three classes: Chlorophyceae, Phaeophyceae, and Rhodophyceae.

Unicellular, colonial, or filamentous.
Grass green due to chlorophyll a and b in chloroplasts.
Chloroplasts vary: discoid, plate-like, reticulate, cup-shaped, spiral, or ribbon-shaped.
Most have pyrenoids (storage bodies with protein and starch) in chloroplasts.
Some store food as oil droplets.
Rigid cell wall: inner cellulose, outer pectose.
E.g., Chlamydomonas, Volvox, Ulothrix, Spirogyra, Chara.

Mostly marine, varying from simple branched, filamentous forms (e.g., Ectocarpus) to profusely branched forms (e.g., kelps, up to 100 m).
Contain chlorophyll a, c, carotenoids, and xanthophylls.
Colour varies from olive green to brown due to fucoxanthin.
Food stored as complex carbohydrates: laminarin or mannitol.
Vegetative cells have a cellulosic wall with a gelatinous algin coating.
Protoplast contains plastids, central vacuole, and nucleus.
Plant body attached by a holdfast, with a stalk (stipe) and leaf-like frond.
E.g., Ectocarpus, Dictyota, Laminaria, Sargassum, Fucus.

Vegetative reproduction: By fragmentation.
Asexual reproduction: By pear-shaped biflagellate zoospores with two unequal, laterally attached flagella.
Sexual reproduction: Isogamous, anisogamous, or oogamous. Gamete union in water or within the oogonium (oogamous species). Gametes are pear-shaped with two laterally attached flagella.

Contain r-phycoerythrin (red pigment).
Mostly marine, especially in warmer areas.
Occur in well-lighted surface waters and deep oceans with low light penetration.
Red thalli are mostly multicellular, some with complex body organisation.
Food stored as floridean starch, similar to amylopectin and glycogen.
E.g., Polysiphonia, Porphyra, Gracilaria, Gelidium.

Vegetative reproduction: By fragmentation.
Asexual reproduction: By non-motile spores.
Sexual reproduction: Oogamous, by non-motile gametes with complex post-fertilisation developments.

Classes	Chlorophyceae (Green Algae)	Phaeophyceae (Brown Algae)	Rhodophyceae (Red Algae)
Major pigments	Chlorophyll a, b	Chlorophyll a, c, Fucoxanthin	Chlorophyll a, d, Phycoerythrin
Stored food	Starch	Mannitol, Laminarin	Floridean Starch
Cell wall	Cellulose	Cellulose and Algin	Cellulose
Flagellar number & position of insertion	2-8, equal, apical	2, unequal, lateral	Absent
Habitat	Fresh water, salt water & brackish water	Fresh water (rare), salt water & brackish water	Fresh water (some), salt water (most) & brackish water