Plant Life Cycles and Alternation of Generations — Revision Notes

⚡ 30-Second Revision

Alternation of Generations — Life cycle with multicellular haploid (gametophyte) and diploid (sporophyte) stages.
Sporophyte (2n) — Diploid, produces haploid spores via meiosis.
Gametophyte (n) — Haploid, produces haploid gametes via mitosis.
Zygote (2n) — Formed by fusion of gametes, develops into sporophyte.
Haplontic — Dominant gametophyte, only zygote is 2n. Zygotic meiosis. Ex: *Spirogyra*.
Diplontic — Dominant sporophyte, only gametes are n. Gametic meiosis. Ex: *Fucus*, Seed plants.
Haplo-diplontic — Both multicellular. Sporic meiosis.

- Bryophytes: Dominant gametophyte, dependent sporophyte. - Pteridophytes: Dominant sporophyte, independent gametophyte. - Gymnosperms/Angiosperms: Dominant sporophyte, highly reduced dependent gametophyte.

Homospory — One type of spore $ightarrow$ bisexual gametophyte.
Heterospory — Two types of spores (micro/mega) $ightarrow$ male/female gametophytes. Prerequisite for seed habit.

2-Minute Revision

Plant life cycles are defined by the alternation of generations, a pattern involving two distinct multicellular phases: the diploid sporophyte and the haploid gametophyte. The sporophyte (2n) produces haploid spores through meiosis.

These spores germinate and develop into the haploid gametophyte (n) via mitosis. The gametophyte then produces haploid gametes (sperm and egg) through mitosis. Fertilization, the fusion of these gametes, forms a diploid zygote (2n), which grows into a new sporophyte.

This cycle ensures both genetic recombination and efficient dispersal.

Three main types exist: Haplontic, where the gametophyte is dominant and the zygote undergoes meiosis (e.g., many algae); Diplontic, where the sporophyte is dominant and gametes are the only haploid stage (e.

g., *Fucus*, seed plants); and Haplo-diplontic, where both are multicellular. In bryophytes, the gametophyte dominates, while in pteridophytes, gymnosperms, and angiosperms, the sporophyte becomes progressively dominant, with the gametophyte becoming reduced and dependent.

This evolutionary trend reflects increasing adaptation to terrestrial environments, particularly the independence from water for fertilization in seed plants.

5-Minute Revision

The core concept of plant reproduction is the alternation of generations, a life cycle strategy where a plant alternates between a multicellular haploid (n) gametophyte and a multicellular diploid (2n) sporophyte. Let's trace the journey:

1
Sporophyte (2n) — This is the diploid generation. Within specialized structures called sporangia, diploid spore mother cells undergo meiosis to produce haploid spores. Meiosis is a reduction division, halving the chromosome number.
2
Spore (n) — These haploid spores are typically dispersed. Upon germination, they develop into the gametophyte through repeated mitotic divisions.
3
Gametophyte (n) — This is the haploid generation. It produces haploid gametes (sperm and egg) within gametangia (antheridia for sperm, archegonia for eggs) through mitosis. Mitosis maintains the haploid chromosome number.
4
Gametes (n) — Male and female gametes fuse during fertilization.
5
Zygote (2n) — The fusion product is a diploid zygote, which is the first cell of the new sporophyte generation. It develops into a multicellular sporophyte through repeated mitotic divisions.

Types of Life Cycles & Examples:

Haplontic — Dominant gametophyte; diploid stage is only the zygote. Zygote undergoes meiosis. Ex: *Volvox*, *Spirogyra*, *Chlamydomonas*.
Diplontic — Dominant sporophyte; haploid stage is only the gametes. Meiosis occurs during gamete formation. Ex: *Fucus*, *Sargassum*, all Gymnosperms and Angiosperms.
Haplo-diplontic — Both sporophyte and gametophyte are multicellular. Meiosis produces spores in the sporophyte.

* Bryophytes (Mosses, Liverworts): Dominant, independent gametophyte; dependent sporophyte. * Pteridophytes (Ferns, Horsetails): Dominant, independent sporophyte; small, independent gametophyte (prothallus). * Gymnosperms & Angiosperms: Dominant sporophyte; highly reduced, dependent gametophyte (e.g., pollen grain, embryo sac).

Key Evolutionary Trends: There's a clear trend towards sporophyte dominance and gametophyte reduction as plants adapted to land. This reduction protects gametes from desiccation and eliminates the need for water for fertilization (via pollen tubes), crucial for terrestrial success.

Prelims Revision Notes

1
Alternation of Generations — The fundamental life cycle pattern in plants, involving a multicellular haploid (gametophyte) stage and a multicellular diploid (sporophyte) stage that alternate.
2
Sporophyte (2n) — Diploid generation. Produces haploid spores via meiosis in sporangia. Develops from a zygote through mitosis.
3
Gametophyte (n) — Haploid generation. Produces haploid gametes via mitosis in gametangia (antheridia/archegonia). Develops from a spore through mitosis.
4
Spore (n) — Haploid reproductive cell from sporophyte (meiosis). Germinates into gametophyte.
5
Gamete (n) — Haploid sex cell from gametophyte (mitosis). Fuses during fertilization.
6
Zygote (2n) — Diploid cell formed by gamete fusion. Develops into sporophyte.
7
Haplontic Life Cycle — Dominant phase is haploid gametophyte. Diploid phase is only the zygote. Meiosis occurs in the zygote (zygotic meiosis). Examples: *Volvox*, *Spirogyra*, *Chlamydomonas* (many algae).
8
Diplontic Life Cycle — Dominant phase is diploid sporophyte. Haploid phase is only the gametes. Meiosis occurs during gamete formation (gametic meiosis). Examples: *Fucus*, *Sargassum* (some algae), all animals, Gymnosperms, Angiosperms.
9
Haplo-diplontic Life Cycle — Both haploid gametophyte and diploid sporophyte are multicellular. Meiosis occurs in the sporophyte to produce spores (sporic meiosis).

* Bryophytes: Gametophyte dominant, independent, photosynthetic. Sporophyte dependent, parasitic on gametophyte. Ex: Mosses, Liverworts. * Pteridophytes: Sporophyte dominant, independent, photosynthetic. Gametophyte small, independent, photosynthetic (prothallus). Ex: Ferns, Horsetails. * Gymnosperms & Angiosperms: Sporophyte overwhelmingly dominant, independent. Gametophyte highly reduced, microscopic, and completely dependent on sporophyte. Ex: Conifers, Flowering plants.

1
Homospory — Production of one type of spore, leading to bisexual gametophytes. Common in most bryophytes and many pteridophytes.
2
Heterospory — Production of two types of spores (microspores $ightarrow$ male gametophyte; megaspores $ightarrow$ female gametophyte). Found in some pteridophytes (*Selaginella*, *Salvinia*) and all seed plants. It is a prerequisite for seed habit.
3
Evolutionary Trend — Progressive reduction of the gametophyte and increasing dominance of the sporophyte, leading to greater adaptation to terrestrial environments (e.g., independence from water for fertilization in seed plants).

Vyyuha Quick Recall

To remember the dominance in Haplo-diplontic cycles: Bryophytes Get Dominant Gametophytes, Pteridophytes Show Dominant Sporophytes. (BGDG, PSDS)