Classification of Plants: Basis and Major Groups

Plants show an astonishing range of body forms -- from a single microscopic cell drifting in a pond, to a mat of moss on a damp wall, to a towering conifer that lives for centuries. Making sense of this diversity requires a scheme that sorts plants by shared, comparable features, not by superficial appearance, so that a classification actually reflects how closely related different plants are.

The criteria used. Botanists build up the classification of the plant kingdom by asking a short sequence of questions about any given plant:

1. Does the plant reproduce by spores, or does it produce true seeds?
2. Does the plant possess specialised vascular tissue -- xylem to conduct water and minerals, phloem to conduct food -- or does it lack this internal transport system?
3. Is the plant body differentiated into recognisable root, stem and leaf regions, or is it a simple, undifferentiated thallus?
4. Where the plant does produce seeds, are those seeds enclosed within a fruit, or do they lie exposed?

The four groups this chapter studies. Applying these questions in sequence sorts plants into the four major groups covered here:

• Algae are mostly aquatic plants whose body is a simple thallus, never differentiated into root, stem and leaves, and which reproduce by spores, never by true seeds.
• Bryophyta are simple land plants with a somewhat more organised body than algae, but still without true vascular tissue; crucially, they still need external water to complete fertilisation.
• Pteridophyta were the first plants, in evolutionary terms, to develop true vascular tissue together with a genuine root, stem and leaves -- yet they still reproduce by spores rather than seeds, so they remain, like algae and bryophytes, spore-bearing plants.
• Gymnosperms are seed-bearing plants, but their seeds are not enclosed within a fruit; they sit exposed, typically on the surface of a cone scale, which is why this group's name literally means 'naked seed'.

A thread that runs through every group. Alongside these four groups, one idea recurs at every stage: every plant's life cycle alternates between a haploid, gamete-producing gametophyte generation and a diploid, spore-producing sporophyte generation. How large, independent and long-lived each of these two generations is differs sharply from one group to the next, and this difference is itself one of the clearest ways to see the evolutionary progression from algae through to gymnosperms -- a theme this chapter returns to in its final section, after the four groups have each been described individually.