Alternation of Generations

The two-body life cycle

Most life cycles you learn first are simple: a diploid adult makes haploid gametes, two gametes fuse, and you are back to a diploid adult. That is the animal cycle. Plants do something stranger — they build two entire multicellular bodies, one with a single set of chromosomes and one with two, and they take turns. The leafy green moss you walk on is haploid. The fern frond, the pine tree, the oak in your yard — those are diploid. The same species lives as both, in sequence. That two-body alternation is the defining feature of every land plant.

The cycle has two reference points and two conversion events. The gametophyte is the haploid (n) body; it makes gametes — egg and sperm — by ordinary mitosis, because its cells are already haploid and fertilization will double the count later. The sporophyte is the diploid (2n) body; it makes spores by meiosis, which cuts the chromosome number in half. Between them sit the two events that flip the ploidy: fertilization (n + n → 2n, a sperm and egg fusing into a zygote) builds the sporophyte, and meiosis (2n → n, inside sporangia) builds the spores that grow the gametophyte. Memorize where meiosis sits and the whole cycle unlocks: plants do sporic meiosis, producing spores, not gametes.

Walking the cycle, step by step

Start with a diploid spore-producing structure called a sporangium on the sporophyte. Inside it, diploid cells called sporocytes undergo meiosis. One sporocyte (2n) produces four haploid spores (n). A spore is not a gamete — it does not need a partner. It germinates and divides by mitosis into a whole new multicellular haploid body: the gametophyte.

On the gametophyte, specialized organs make gametes by mitosis. In mosses and ferns these are the antheridium (producing flagellated sperm that swim through a film of water) and the archegonium (housing a single egg). When a sperm reaches an egg, fertilization fuses them into a diploid zygote (2n). The zygote does not make spores immediately — it divides by mitosis into the multicellular diploid sporophyte. That sporophyte grows sporangia, meiosis happens again, and the loop closes. The key asymmetry: meiosis happens once per turn (sporophyte → spores) and fertilization happens once per turn (gametes → zygote), and a whole multicellular body is built between each pair of events.

Two specialized vocabulary terms tidy this up. A plant is homosporous if it makes one kind of spore that grows into a bisexual gametophyte (most ferns, many bryophytes). It is heterosporous if it makes two sizes: small microspores that grow into male gametophytes and large megaspores that grow into female gametophytes (all seed plants, plus lycophytes like Selaginella and water ferns like Marsilea). Heterospory is the evolutionary prerequisite for seeds and pollen.

How seed plants miniaturized the gametophyte

In a fern, the gametophyte is a free-living green heart-shaped prothallus a few millimeters wide that photosynthesizes on its own. In seed plants the gametophyte is reduced almost to invisibility and never leaves the parent sporophyte. In a flowering plant, two miniature gametophytes form inside the flower, both on the diploid sporophyte:

• The male gametophyte is the pollen grain. In the anther, diploid microsporocytes undergo meiosis to make four haploid microspores; each divides mitotically into a pollen grain of just three cells — one tube cell and two sperm cells. The entire male "generation" is three cells riding the wind or an insect.
• The female gametophyte is the embryo sac. In the ovule, a single megasporocyte undergoes meiosis to make four haploid megaspores; three usually degenerate and one survives, dividing three times to form the embryo sac — typically seven cells with eight nuclei, including the egg cell and the binucleate central cell.

Pollination delivers the pollen grain to the stigma; the tube cell grows a pollen tube down the style and delivers the two sperm. Then angiosperms do their signature trick — double fertilization. One sperm fuses with the egg to form the diploid (2n) zygote, the next sporophyte. The second sperm fuses with the two polar nuclei to form a triploid (3n) endosperm, the nutritive tissue that becomes the starchy interior of grains. The egg fusion is the real generation-closing event; the endosperm is an extra angiosperm innovation. Gymnosperms do single fertilization and feed the embryo with haploid female-gametophyte tissue instead.

The players and conversions

• Sporophyte (2n). The diploid body. Site of meiosis. In ferns and seed plants it is the large, long-lived, photosynthetic plant you recognize.
• Gametophyte (n). The haploid body. Site of gamete production by mitosis. Dominant in mosses; reduced to microscopic in seed plants.
• Spore (n). A meiotic product that develops alone, by mitosis, into a gametophyte. Spores divide.
• Gamete (n). A mitotic product of the gametophyte that must fuse to develop. Gametes fuse.
• Sporangium. The diploid organ where meiosis makes spores (the fern's sori, the moss capsule, the anther and ovule of a flower).
• Zygote (2n). The product of fertilization; first cell of the new sporophyte.
• Meiosis (2n → n). The halving step. Always happens in the sporophyte. Defines plant cycles as "sporic."
• Fertilization (n + n → 2n). The doubling step. Fuses gametes into the zygote.

Dominant phase across plant groups

Group	Dominant phase	Gametophyte form	Sporophyte form	Spore type	Sperm transport
Liverworts / hornworts	Gametophyte (n)	Flat thallus or leafy shoot	Small dependent capsule	Homosporous	Swimming (water film)
Mosses	Gametophyte (n)	Leafy green carpet	Stalk + capsule on gametophyte	Homosporous	Swimming (water film)
Ferns	Sporophyte (2n)	Free-living prothallus (~mm)	Large leafy fronds	Mostly homosporous	Swimming (water film)
Selaginella (lycophyte)	Sporophyte (2n)	Tiny, inside spore wall	Branching shoot	Heterosporous	Swimming (water film)
Gymnosperms (pine)	Sporophyte (2n)	Pollen grain + ovule tissue	Tree	Heterosporous	Pollen tube (mostly)
Angiosperms (oak, grass)	Sporophyte (2n)	3-cell pollen + 7-cell embryo sac	Flowering plant	Heterosporous	Pollen tube

The figures, in real units

• 470 million years of reduction. Land plants arose from charophyte algae roughly 470 Mya. Over that span the gametophyte went from the dominant, free-living body (still true in bryophytes) to a 3-cell pollen grain — a reduction in cell count of several orders of magnitude relative to a meters-tall sporophyte.
• Pollen grain: 3 cells. The mature male gametophyte of a flowering plant is a tube cell plus two sperm cells. Pollen grains range from about 6 µm (forget-me-not) to over 100 µm (some cucurbits); most cluster around 25–50 µm.
• Embryo sac: 7 cells, 8 nuclei. The canonical Polygonum-type female gametophyte has one egg, two synergids, three antipodal cells, and one binucleate central cell.
• 4 spores per meiosis. Each diploid sporocyte yields four haploid products. In the ovule three megaspores typically degenerate so only one builds the embryo sac.
• Sporophyte dominance ratio. In an oak (tens of meters, billions of cells) versus its 3-cell pollen, the diploid body outweighs the haploid by a factor of perhaps 10⁹ or more — the most extreme sporophyte dominance on the tree of life.
• Endosperm ploidy: 3n. Double fertilization produces a triploid endosperm; this nutritive tissue is the bulk of a wheat, rice, or maize kernel and supplies a large share of human dietary calories.
• Bryophyte sporophyte lifespan. A moss sporophyte is short-lived — weeks to months — and never becomes nutritionally independent of the gametophyte it grows from, unlike the decades-long sporophyte of a tree.

Where it shows up — real organisms and examples

• The lawn moss you ignore is the haploid generation. When a moss "goes to seed," the brown stalks with capsules poking above the green carpet are the entire diploid sporophyte generation, parasitic on the haploid plant below.
• Fern "dust" on the underside of fronds. The brown dots (sori) on a fern leaf are clusters of sporangia. The spores they release grow into the rarely-noticed heart-shaped prothallus — a free-living, photosynthetic, haploid plant most people never see.
• Hay fever is gametophyte exposure. Pollen that triggers seasonal allergies is the airborne male gametophyte generation of grasses, ragweed, and trees — a haploid organism, not just a "cell."
• Endosperm feeds the world. The flour in bread, the white of polished rice, the body of a maize kernel — all are 3n endosperm produced by the angiosperm's double-fertilization version of the cycle's fertilization step.
• Sea lettuce (Ulva) shows isomorphic alternation. Unlike plants, this green alga has a gametophyte and sporophyte that look identical (isomorphic) — a reminder that the two phases need not differ in form, only in ploidy.
• Kelp and other brown algae alternate too. Oarweed (Laminaria) has a huge diploid sporophyte and a microscopic filamentous gametophyte — independent evolution of the same sporophyte-dominant pattern seen in plants.

Common misconceptions and pitfalls

• "Plants make gametes by meiosis." No — that is the animal rule. Plants make spores by meiosis and gametes by mitosis. Mixing this up is the single most common error. The gametophyte is already haploid, so it just divides its cells to make egg and sperm.
• "The big visible plant is always diploid." True for ferns and seed plants, false for mosses. In a moss the conspicuous green plant is the haploid gametophyte; the diploid sporophyte is the small stalk on top.
• "A spore and a seed are the same thing." A spore is a single haploid cell that grows into a gametophyte. A seed is a multicellular package — a diploid embryo (young sporophyte) plus stored food (endosperm or gametophyte tissue) wrapped in a coat. Seeds are an angiosperm/gymnosperm structure built after fertilization.
• "Pollen is sperm." Pollen is the entire male gametophyte (3 cells), which contains two sperm cells. The sperm are delivered by the pollen tube; the pollen grain itself is a tiny multicellular organism, not a gamete.
• "Alternation of generations means switching between asexual and sexual." No — it refers to alternating haploid and diploid multicellular bodies within a sexual cycle. Both phases can be produced asexually, but the term is about ploidy, not the sex/asex distinction.
• "Endosperm is part of the embryo." Endosperm (3n) is a separate nutritive tissue formed by the second sperm fusing with the central cell; the embryo (2n) is formed by the first sperm fusing with the egg. They are genetically and developmentally distinct, which is why genomic imprinting tightly regulates endosperm.