Unit 2: Adaptation of Land Plants

Syllabus Reference: Salient Features; Adaptations to land habit, Amphibian nature of Bryophyte, Evolution of Land Plants.

1. Salient Features of Land Plants

Land plants (Embryophytes) generally share these core characteristics:

• Multicellular Eukaryotes: Complex cellular organization.
• Photosynthetic: Use Chlorophyll a and b.
• Cell Walls: Made of cellulose.
• Cuticle: A waxy covering to prevent water loss.
• Stomata: Pores for gas exchange.
• Embryo Retention: The fertilized egg (zygote) develops into an embryo within the tissue of the female parent.

2. Adaptations to Land Habit

For plants to move from an aquatic environment to a terrestrial one, they developed specific adaptations:

A. Conservation of Water

• Epidermis & Cuticle: The outer layer of cells is covered by a waxy layer called the cuticle that makes the plant waterproof, preventing desiccation (drying out).
• Stomata: Specialized openings that can open and close to regulate water loss (transpiration) while allowing CO₂ entry.

B. Absorption and Transport

• Rhizoids/Roots: Bryophytes use rhizoids, while vascular plants use true roots to anchor in soil and absorb water/minerals.
• Vascular Tissue: Evolution of Xylem (water transport) and Phloem (food transport) allowed plants to grow tall and transport nutrients efficiently (seen in Pteridophytes and Gymnosperms).

C. Mechanical Support

• Air provides less support than water. Land plants developed Lignin (a hard substance) in their cell walls (especially in xylem) to stand upright against gravity.

3. Amphibian Nature of Bryophytes

Bryophytes are famously known as the "Amphibians of the Plant Kingdom." This is a crucial concept in Botany.

Why are they called Amphibians?

1. Land Dwellers: The vegetative plant body lives on land (soil, rocks, tree trunks).
2. Water Dependence: They are dependent on external water (rain, dew) for sexual reproduction.

Mechanism:

The male gametes (antherozoids) are flagellated and motile. They must swim through a thin film of water to reach the archegonium (female organ) to fertilize the egg. Without water, fertilization cannot occur, and the life cycle cannot complete. This restricts bryophytes to moist, shady habitats.

4. Evolution of Land Plants

The evolution of land plants is believed to have occurred from Green Algae (Chlorophyta), specifically the Charophytes.

Evidences of Algal Origin:

• Both contain photosynthetic pigments Chlorophyll a and b.
• Both store food as starch.
• Both have cell walls made of cellulose.
• Flagellated sperm structure is similar.

The Evolutionary Sequence:

Algae → Bryophytes (First land plants, non-vascular) → Pteridophytes (First vascular plants, seedless) → Gymnosperms (Naked seeds) → Angiosperms (Fruits/Flowers).