Plants move onto land
Land plants evolved from green algae and first appear in the fossil record about 470 million years ago. Life on land offered bright light and plenty of carbon dioxide, but brought new problems: drying out, holding the body up without the support of water, and getting sperm to eggs. Early land plants had a waxy cuticle, spores with tough walls, and protected their embryos inside the parent plant. That last feature is why land plants are also called embryophytes.
The four main groups
| Group | Key features | Examples |
|---|---|---|
| Bryophytes | No vascular tissue; spores; gametophyte dominant | Mosses, liverworts, hornworts |
| Ferns and allies | Vascular tissue; spores; sporophyte dominant | Ferns, horsetails, clubmosses |
| Gymnosperms | Seeds on cone scales; pollen | Pines, spruces, cycads, ginkgo |
| Angiosperms | Flowers; seeds inside fruits | Grasses, oaks, roses, orchids |
Bryophytes
Mosses, liverworts and hornworts (about 20,000 species) have no lignified vascular tissue. Water moves from cell to cell or over the surface, so they stay small, usually only a few centimetres tall. Root-likerhizoids anchor them but are not true roots. The green plant is the haploidgametophyte. Its sperm swim through rain or dew to the eggs, and the resulting sporophyte, a stalk and capsule, grows on top of the gametophyte and depends on it.
Ferns and their allies
About 430 million years ago, plants evolved vascular tissue: xylem with walls strengthened bylignin, and phloem. Plants could now carry water from true roots to leaves held high in the air, and the first forests grew. Ferns, horsetails and clubmosses (about 13,000 species) still reproduce withspores. The fern plant is the diploid sporophyte; brown sori under the fronds release spores, which grow into a tiny, independent gametophyte, the prothallus. Its sperm, too, must swim.
Gymnosperms
Seed plants appeared about 360 million years ago. Their male gametophytes are packed into pollengrains carried by the wind, so fertilisation no longer needs a film of water. A seed contains an embryo, a food store and a protective coat. In gymnosperms (“naked seeds”; about 1,100 species, including pines, spruces, cycads and ginkgo), the seeds sit on the surface of cone scales rather than inside a fruit. Conifers dominate cold northern forests and include the tallest and oldest living trees.
Angiosperms
Flowering plants appeared about 140 million years ago and now make up roughly 90% of land plant species, around 300,000 in all. Their ovules are enclosed in an ovary, which develops into a fruitthat protects and spreads the seeds. Flowers attract insects, birds and bats that carry pollen accurately from flower to flower. Grasses and most broad-leaved trees, such as oaks, are flowering plants with small, wind-pollinated flowers. How pollination and double fertilisation work is explored in the Plant Reproduction topic.
Alternation of generations
Every land plant’s life cycle alternates between two multicellular generations:
- The gametophyte is haploid (n). It makes gametes (sperm and eggs) by mitosis.
- Two gametes fuse at fertilisation to make a diploid (2n) zygote, which grows into the sporophyte.
- The sporophyte makes haploid spores by meiosis, and each spore grows into a new gametophyte.
The balance between the generations shifted during evolution. In mosses the gametophyte is the main plant. In ferns the sporophyte is the main plant but the gametophyte still lives independently. In seed plants the gametophytes are reduced to a few cells hidden inside pollen grains and ovules, protected by the sporophyte.
Trends in plant evolution
- From small plants without vascular tissue to tall plants with lignified xylem.
- From a dominant gametophyte to a dominant sporophyte.
- From swimming sperm to pollen, freeing reproduction from water.
- From single-celled spores to seeds, and then to seeds protected inside fruits.
Why it matters
- Food: almost all crops, including wheat, rice and maize (all grasses), are flowering plants.
- Timber and paper: much of it comes from conifers.
- Coal: formed from the remains of giant clubmoss and horsetail forests of the Carboniferous period, around 300 million years ago.
- Peat bogs: built by sphagnum moss, they store a huge amount of carbon.
Common misconceptions
- “Clubmosses, Spanish moss and reindeer moss are mosses.” They are a vascular plant, a flowering plant and a lichen.
- “Trees are not flowering plants.” Oaks, maples and most broad-leaved trees flower; conifers do not.
- “Spores and seeds are the same.” A spore is a single cell; a seed contains a multicellular embryo with food and a coat.
- “Pollen is sperm.” A pollen grain is a tiny male gametophyte that produces sperm cells.
- “Ferns have seeds on their leaves.” The brown dots are sori, which release spores.
What the simulator simplifies
The tree shows only four broad groups and a few key innovations; real plant relationships are more detailed, and flowering plants and gymnosperms evolved from extinct seed-plant ancestors not shown here. Dates are approximate first appearances in the fossil record and species counts are rounded estimates. The life cycles are summarised in five or six stages, and drawings are not to scale. In the water experiment the threshold for a “continuous water film” is arbitrary, the flower is assumed to be insect-pollinated, and the pine is pollinated only by wind. Some gymnosperms (cycads and ginkgo) actually still have swimming sperm, released inside the ovule.
Key takeaways
- Land plants evolved from algae about 470 million years ago and split into four main groups.
- Vascular tissue let plants grow tall; seeds and pollen freed them from water; flowers and fruits brought animal partners.
- All land plants alternate between a haploid gametophyte and a diploid sporophyte.
- The gametophyte is dominant in mosses; the sporophyte is dominant in ferns and seed plants.
- Mosses and ferns need water for their swimming sperm; seed plants use pollen.