Alternation of generations

1The fern's hidden second life

Turn over a fern frond on a damp forest bank and you may see rows of brown or orange dots underneath. They are not a disease: each dot is a cluster of tiny capsules full of spores. A spore that lands on wet soil does not grow into another fern. It grows into a flat green heart, smaller than your fingernail, that most people never notice. Only this little plant can make sperm and eggs; the familiar fern grows from the fertilised egg, on top of it.

So a fern species lives as two different-looking plants, one following the other, generation after generation. All land plants share this pattern, called alternation of generations, but the balance between the two generations has shifted during evolution, and that shift explains how plants conquered dry land.

2The general life cycle

Notice where each type of division happens. Unlike animals, plants do not make gametes by meiosis: meiosis makes spores, and gametes are made by mitosis in a body that is already haploid. Fertilisation then restores the diploid number. The two generations differ in chromosome number, appearance and job: the sporophyte disperses the species by spores; the gametophyte brings about sexual reproduction.

3Mosses: the gametophyte is the plant you see

In a moss (a bryophyte), the soft green cushion is a mass of leafy gametophytes, anchored by rhizoids. Antheridia and archegonia form at the tips of the shoots. Sperm have flagella and must swim through a film of rain or dew to an archegonium, so mosses need water to reproduce. The zygote grows into a small sporophyte, a capsule on a stalk (seta), which stays attached to the gametophyte and depends on it for water and minerals. Spores are made by meiosis in the capsule and released when its lid falls off.

4Ferns: the sporophyte takes over

In a fern the large, long-lived plant with fronds, rhizome and roots is the sporophyte. It has true xylem and phloem, so it can grow tall and live independently of the gametophyte. Spores are made by meiosis in sporangia grouped into sori on the underside of the fronds. Each spore grows into a prothallus, a small, thin, heart-shaped gametophyte that lives on its own for a few weeks. Its archegonia and antheridia are on its lower surface, and sperm again need water to swim to the eggs. The young sporophyte grows out of the archegonium, and the prothallus soon dies.

5Flowering plants: gametophytes hidden in the flower

In a flowering plant the whole visible plant is the sporophyte. The gametophytes are reduced to a few cells and never live freely:

• In the anther, meiosis makes microspores; each divides by mitosis to become a pollen grain, the male gametophyte, with just two or three cells.
• In each ovule, meiosis makes a megaspore; three rounds of mitosis turn it into the embryo sac, the female gametophyte: seven cells with eight nuclei, one of which is the egg.

Pollen is carried to the flower by wind or animals, and the male gametes travel to the egg inside a pollen tube, so fertilisation needs no external water. The fertilised ovule becomes a seed: a young sporophyte (the embryo) with a food store, wrapped in a protective coat. Lesson 4 follows these steps in detail.