This chapter tracks the “Green Evolution”—how life moved from the oceans to the land, developing roots, vascular systems, and eventually the seeds and flowers that dominate our world today.

The Green Revolution: Mastering the Plant Kingdom

Plants are more than just background scenery. They are the primary producers of all energy on land. In this chapter, we move beyond “it’s green and has leaves” to understanding the evolutionary milestones that separate a simple seaweed from a massive Redwood tree.

We classify the kingdom into five major groups: Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.

The Core Pillars of Plant Classification

1. Algae: The Pioneers

Algae are chlorophyll-bearing, simple, thalloid organisms. They are mostly aquatic.

• Chlorophyceae: Green algae (stored food: starch).
• Phaeophyceae: Brown algae (stored food: mannitol/laminarin).
• Rhodophyceae: Red algae (stored food: floridean starch).

2. Bryophytes: The Amphibians of the Plant Kingdom

These include Mosses and Liverworts. They live in soil but depend on water for sexual reproduction (their sperm must swim to the egg).

• Key Feature: The dominant phase is the Gametophyte (haploid).

3. Pteridophytes: The First Land Conquerors

These are the first terrestrial plants to possess vascular tissues (Xylem and Phloem).

• Key Feature: The dominant phase is the Sporophyte (diploid). They produce spores instead of seeds.

4. Gymnosperms: The Naked Seeds

These plants have “naked” seeds because their ovules are not enclosed by an ovary wall.

• Key Examples: Pinus and Cycas. They are known for surviving extreme environments.

5. Angiosperms: The Flowering Giants

The most advanced group. Seeds are enclosed in fruits, and pollination occurs through flowers. They undergo a unique process called Double Fertilization.

The Gauntlet: 10 Challenging Aptitude Questions

Question 1: The Algal “Store”

A researcher finds an alga that stores its food as a complex carbohydrate very similar to amylopectin and glycogen in structure. To which class does this alga belong?

Question 2: The Bryophyte Dependency

Why are Bryophytes limited in their height and usually found in moist, shady habitats? Give two biological reasons.

Question 3: Prothallus Logic

In Pteridophytes, what is a “Prothallus,” and why does its requirement for water limit the geographical spread of these plants?

Question 4: The Mycorrhiza Link

In Gymnosperms, which specific genus has a symbiotic association with fungi in the form of mycorrhiza, and which genus has specialized “coralloid roots”?

Question 5: Double Fertilization

In Angiosperms, one male gamete fuses with the egg, and the other fuses with the secondary nucleus. What are the resulting structures of these two fusions?

Question 6: Diplontic vs. Haplontic

Most Algae show a Haplontic life cycle. However, identify the Alga that is Diplontic and the one that is Haplo-diplontic.

Question 7: The “Peat” Producer

Which Moss is responsible for the formation of “Peat,” which is used as fuel and packing material for trans-shipment of living material?

Question 8: Heterospory

What is “Heterospory,” and why is it considered an important step toward the “Seed Habit” in evolution? Name two Pteridophytes that show this.

Question 9: The Gymnosperm Needle

Gymnosperms like Conifers have needle-like leaves, thick cuticles, and sunken stomata. What is the physiological purpose of these adaptations?

Question 10: The Triple Fusion

Why is the endosperm in Angiosperms triploid (3n)? Show the mathematical fusion of nuclei.

Detailed Explanations & Solutions

1. The Algal Store

Floridean starch (found in Red Algae) is structurally very similar to amylopectin and glycogen.

Result: Rhodophyceae.

2. Bryophyte Height

1. They lack vascular tissues (Xylem/Phloem) to transport water to great heights. 2. They need a film of water for their flagellated male gametes (antherozoids) to reach the archegonium.Result: Lack of vascularity and water-dependency for sex.

3. Prothallus

The prothallus is the inconspicuous, small, multicellular, free-living gametophyte of Pteridophytes.

Result: It needs cool, damp, shady places and water for fertilization, restricting Pteridophytes to specific narrow geographical zones.

4. Gymnosperm Roots

• Pinus: Associated with Mycorrhiza.
• Cycas: Has Coralloid roots associated with Nitrogen-fixing cyanobacteria.Result: Pinus (Fungi) and Cycas (Cyanobacteria).

5. Double Fertilization Products

1. Male Gamete + Egg = Zygote (2n).
2. Male Gamete + Secondary Nucleus = Primary Endosperm Nucleus (3n).Result: Zygote and Endosperm.

6. Life Cycle Exceptions

• Fucus: Diplontic.
• Ectocarpus / Polysiphonia / Kelps: Haplo-diplontic.Result: Fucus is the big exception for Algae.

7. Peat Producer

Sphagnum (a moss) provides peat. It has great water-holding capacity.

Result: Sphagnum.

8. Heterospory

Production of two kinds of spores (Macro and Micro). The development of the zygote into an embryo takes place within the female gametophyte, which is retained on the parent sporophyte.

Result: Selaginella and Salvinia. This is the precursor to the seed habit.

9. Conifer Adaptations

These are all “Xerophytic” adaptations.

Result: To reduce water loss due to transpiration in extreme cold or dry conditions.

10. Triple Fusion

One haploid male gamete (n) + two haploid polar nuclei (n+n).

Result: n + n + n = 3n (Triploid Endosperm).

Pro-Tip: The Evolution Hierarchy

Think of the groups as a staircase of complexity:

• Algae: No body differentiation.
• Bryophytes: No vascular tissue (Roots/Stem/Leaves start appearing).
• Pteridophytes: Vascular tissue appears (Spores only).
• Gymnosperms: Seeds appear (but “naked”).
• Angiosperms: Flowers and Fruit appear (Seeds protected).