Alternation of Generations in Plant Groups with exception

🌿 Alternation of Generations in Plant Groups

Bryophyta • Pteridophyta • Gymnosperms • Angiosperms

Alternation of generations is the life cycle in which plants alternate between a haploid gametophyte (n) and a diploid sporophyte (2n).

All four groups show heteromorphic alternation of generations, but the dominant generation changes during evolution.

🌍Bryophyta (Mosses & Liverworts)

🔹 Dominant generation: Gametophyte (n)

🔹 Sporophyte: Dependent on gametophyte

Details:

Green, leafy/thalloid plant = gametophyte

Sex organs: Antheridia & archegonia

Sporophyte differentiated into foot, seta, capsule

Spores formed by meiosis in capsule

Fertilization requires water

📌 Example: Funaria, Marchantia

🌱Pteridophyta (Ferns & Allies)

🔹 Dominant generation: Sporophyte (2n)

🔹 Gametophyte: Independent (prothallus)

Details:

Fern plant = sporophyte

Sporangia on sporophylls produce spores

Spores form prothallus (gametophyte)

Prothallus bears antheridia & archegonia

Fertilization requires water

📌 Example: Pteris, Dryopteris

🌲Gymnosperms

🔹 Dominant generation: Sporophyte (2n)

🔹 Gametophyte: Highly reduced & dependent

Details:

Plant body is sporophyte

Male & female cones present

Male gametophyte = pollen grain

Female gametophyte inside ovule

Fertilization not water-dependent

Seeds are naked

📌 Example: Pinus, Cycas

🌹Angiosperms (Flowering Plants)

🔹 Dominant generation: Sporophyte (2n)

🔹 Gametophyte: Extremely reduced

Details:

Flowering plant = sporophyte

Male gametophyte = pollen grain

Female gametophyte = embryo sac

Double fertilization present

Seeds enclosed in fruit

📌 Example: Sunflower, Pea

⚠️ Exceptions in Alternation of Generations

• Bryophytes • Pteridophytes • Gymnosperm • Angiosperms

Although plants show a typical alternation of generations.

🌱 Exceptions in Bryophytes

🔹 Normal rule

Gametophyte dominant

Sporophyte dependent

⚠️ Exception

Sporophyte shows partial independence

Example: Anthoceros (Hornwort)

Sporophyte is:

Green

Photosynthetic

Long-lived

Can manufacture some of its own food

📌 Still dependent, but least dependent sporophyte among bryophytes

🌿 Exceptions in Pteridophytes

🔹 Normal rule

Sporophyte dominant

Gametophyte (prothallus) free-living

⚠️ Exceptions

(a) Dependent Gametophyte

Examples:

Selaginella

Isoetes

➡️ Gametophyte develops inside the spore wall

➡️ Called endosporic gametophyte

(b) Heterospory (Exception to homospory)

Examples:

Selaginella

Salvinia

Marsilea

➡️ Produce:

Microspores → male gametophyte

Megaspores → female gametophyte

📌 Important evolutionary step towards seed habit

🌲Exceptions in Alternation of Generations in Gymnosperms

Gymnosperms generally show a heteromorphic alternation of generations with a dominant sporophyte. However, some exceptions and special features are important for exams:

1. Highly Reduced Gametophytes (Exceptional Feature)

Male gametophyte → represented by only 2–3 cells (pollen grain).

Female gametophyte → remains retained within the megasporangium (ovule).

👉 No free-living gametophyte (unlike pteridophytes).

📌 Exception: Alternation of generations is not clearly visible.

2. Endosperm is Haploid (n) – Not Triploid

In gymnosperms, the endosperm is formed before fertilization.

It is actually the female gametophyte tissue (n).

📌 Exception compared to angiosperms:

Gymnosperms → haploid endosperm (n)

Angiosperms → triploid endosperm (3n)

3. Archegonia Absent in Some Gymnosperms

Archegonia present in most gymnosperms (Pinus, Cycas).

Absent in Gnetales:

Gnetum

Welwitschia

📌 Exception: Female gametophyte lacks archegonia.

4. Polyembryony (More Than One Embryo)

Common in gymnosperms like Pinus.

Multiple embryos develop from a single ovule.

📌 Exception: Deviates from normal single embryo formation.

5. Motile Male Gametes in Cycas & Ginkgo

Male gametes are large and motile (flagellated).

Most gymnosperms have non-motile male gametes.

📌 Exception within gymnosperms:

Motile sperm → Cycas, Ginkgo

Non-motile sperm → Pinus, Gnetum

6. Double Fertilization-like Condition (Gnetum)

Gnetum shows two fertilization events.

But no triploid endosperm is formed.

📌 Exception: Resembles angiosperms but incomplete.

Note:

Gymnosperms show highly reduced, dependent gametophytes, haploid endosperm, and special exceptions like motile sperm (Cycas, Ginkgo) and absence of archegonia in Gnetales.

🌸  Exceptions in Angiosperms

🔹 Normal rule

Sporophyte dominant

Gametophyte highly reduced

⚠️ Exceptions / Special Features

(a) No free-living gametophyte

Gametophytes are:

Completely dependent

Microscopic

📌 Male gametophyte → Pollen grain

📌 Female gametophyte → Embryo sac

(b) Double Fertilization (Unique Exception)

One male gamete + egg → zygote (2n)

Other male gamete + polar nuclei → endosperm (3n)

📌 Seen only in angiosperms