Systems of plant classification

Kingdom Plantae now holds five groups, from simple to complex: algae, bryophytes, pteridophytes, gymnosperms and angiosperms. Fungi, and the walled members of Monera and Protista, are no longer counted as plants. Cyanobacteria (blue-green algae) are not called algae any more; they are bacteria.

Algae belong here because they share the basic plant features: they photosynthesise, their cells are eukaryotic, and they have cellulose cell walls.

The ways of classifying plants changed over time.

• Artificial systems, like that of Linnaeus, used only a few characters you can easily see: the plant's habit, colour, number and shape of leaves, and the androecium (stamens). They gave equal weight to vegetative parts (leaf, stem, root) and sexual parts. But vegetative parts change easily with weather and surroundings, so closely related plants sometimes ended up in different groups.
• Natural systems looked at natural affinities: external features and also internal ones, such as cell ultrastructure, anatomy, embryology and the plant's chemicals (phytochemistry). George Bentham and Joseph Dalton Hooker gave such a system for flowering plants.
• Phylogenetic systems are based on evolutionary relationships. They assume that members of the same group share a common ancestor.

Some newer tools help when there is doubt:

• Numerical taxonomy: every observable character is given a number or code and a computer compares them all, giving equal importance to each character. Hundreds of characters can be handled at once.
• Cytotaxonomy: uses chromosome number, structure and behaviour.
• Chemotaxonomy: uses the chemical constituents of the plant.