Summary

The plant kingdom brings together five great groups — the algae, the bryophytes, the pteridophytes, the gymnosperms and the angiosperms. Read in that order, they tell a story of plants gradually moving out of the water, growing more complex in their bodies, developing better ways to move water and nutrients, and slowly cutting their dependence on water for reproduction.

The algae are the simplest of these. They are chlorophyll-bearing, autotrophic organisms with a thalloid body that shows little internal differentiation, and they live largely in water. On the basis of the pigments they contain and the kind of food they store, algae are sorted into three classes — the green Chlorophyceae, the brown Phaeophyceae and the red Rhodophyceae. They reproduce in several ways: vegetatively by fragmentation, asexually by producing various kinds of spores, and sexually by forming gametes, whose fusion may be isogamous, anisogamous or oogamous depending on how similar the two gametes are.

The bryophytes represent the first move onto land, yet they remain tied to water. They can grow in soil but still need water to carry out sexual reproduction. Their body is more differentiated than an alga's — it is thallus-like, either flat and spreading or upright, and it clings to the surface it grows on by rhizoids, bearing structures that look like roots, stems and leaves without being true ones. Bryophytes fall into the liverworts, whose body is a flat, dorsiventral thallus, and the mosses, which have slender upright axes clothed in spirally set leaves. In a bryophyte the conspicuous plant is the gametophyte: it produces the sex organs — antheridia that make male gametes and archegonia that hold the female gamete — and after these gametes fuse, the resulting zygote grows into a sporophyte that produces haploid spores. Those spores then germinate to form new gametophytes.

In the pteridophytes the balance shifts. Here the dominant, conspicuous plant is the sporophyte, and for the first time it is divided into a true root, stem and leaves, all served by well-developed vascular tissues that conduct water and food. This sporophyte bears sporangia that release spores; the spores germinate into small gametophytes that need cool, damp places to survive. On these gametophytes sit the antheridia and archegonia, and water is still required to ferry the male gametes to the egg in the archegonium, where fertilisation forms a zygote that grows into the next sporophyte.

The gymnosperms take the plant body further from water. In them the ovules are never wrapped in an ovary wall, so after fertilisation the seeds lie exposed — the reason they are called naked-seeded plants. They are heterosporous, producing microspores and megaspores in microsporangia and megasporangia that are carried on sporophylls; the microsporophylls and megasporophylls are arranged in spirals to form male and female cones. The pollen grain germinates and its pollen tube delivers the male gamete right into the ovule, where it fuses with the egg in the archegonium. With this, fertilisation no longer depends on external water. After fusion the zygote becomes an embryo and the ovule becomes a seed.

Finally, the angiosperms complete the trend. Their ovules and pollen are produced inside flowers, and their seeds are enclosed within fruits rather than left bare. This group is split into two classes, the dicotyledons and the monocotyledons. …