Alternation of Generations and Land Plant Diversity

Core Idea

All land plants have a life cycle called alternation of generations, in which a multicellular haploid stage (the gametophyte) alternates with a multicellular diploid stage (the sporophyte). The sporophyte makes haploid spores by meiosis, and each spore grows by mitosis into a gametophyte, which makes gametes by mitosis. The major plant groups differ in which generation is larger and longer-lived: the gametophyte dominates in mosses and other bryophytes, while the sporophyte dominates in ferns and seed plants. Over plant evolution the gametophyte became smaller and more protected, ending up usually microscopic and enclosed in the tissues of seed plants.

How It's Best Learned

Draw one general cycle with a vertical line separating haploid (n) from diploid (2n), marking where meiosis and fertilization cross the line. Then redraw it three times, for a moss, a fern and a flowering plant, scaling each generation by size. Real specimens help: a moss with brown sporophyte stalks and a fern frond with sori on its underside.

Common Misconceptions

• Plants make gametes by meiosis, as animals do. (In plants, meiosis makes spores; gametes are made later by mitosis in the haploid gametophyte.)
• The green, leafy moss plant is diploid. (The leafy moss is the haploid gametophyte; the diploid sporophyte is the stalk and capsule that grows from it.)
• Pollen grains are sperm. (A pollen grain is a tiny male gametophyte that produces sperm cells.)

Explainer

Animals have one multicellular generation. A diploid body makes haploid gametes by meiosis, and fertilization restores the diploid state. Land plants have two multicellular generations that take turns.

The diploid generation is the sporophyte. Inside structures called sporangia, cells undergo meiosis and produce haploid spores. A spore does not fuse with anything. It divides by mitosis and grows into the haploid generation, the gametophyte. The gametophyte makes eggs and sperm by mitosis, since its cells are already haploid. When a sperm fertilizes an egg, the diploid zygote grows by mitosis into a new sporophyte. So the two transitions between haploid and diploid are meiosis (sporophyte to spore) and fertilization (gametes to zygote).

Land plants evolved from green algae called charophytes, and the major groups show different versions of this cycle.

Bryophytes, which include mosses, liverworts and hornworts, have no true vascular tissue. Their gametophyte is the familiar green plant. The sporophyte is a stalk with a spore capsule that stays attached to the gametophyte and depends on it for food. Bryophyte sperm have flagella and swim through water to the egg.

Seedless vascular plants, such as ferns and lycophytes, reversed the balance. The large fern plant with fronds is the sporophyte. It has xylem and phloem, which let it grow tall. Clusters of sporangia called sori form on the undersides of fronds. Spores grow into small, heart-shaped gametophytes that live on their own on damp soil. Fern sperm still swim, so ferns need water for fertilization.

Seed plants, the gymnosperms and the flowering plants, carried the trend further. The sporophyte is the whole visible plant. The gametophytes are usually microscopic and develop inside sporophyte tissue. The male gametophyte is the pollen grain, which travels by wind or animals and delivers sperm through a pollen tube, so no free water is needed. The female gametophyte develops inside an ovule. After fertilization the ovule becomes a seed, which holds an embryo, a food supply and a protective coat.

Read across the groups, the pattern is a steady shift from a dominant gametophyte to a dominant sporophyte. The diploid stage carries two copies of each gene, which may help mask harmful mutations. The changes also match the move onto dry land: vascular tissue, pollen and seeds each reduced a plant's dependence on water.