Introduction: Classifying the Plant Kingdom

The previous chapter looked at how living organisms as a whole are grouped, following the Five Kingdom system proposed by R.H. Whittaker in 1969, which sorts all organisms into Monera, Protista, Fungi, Animalia and Plantae. This chapter zooms into just one of those five — Kingdom Plantae, informally the 'plant kingdom' — and looks at how it is further divided.

It helps to know that the boundary of Plantae has itself shifted over time. Older classifications lumped fungi, and any member of Monera or Protista that happened to have a cell wall, in with the plants. That grouping is no longer used: fungi and the walled members of Monera and Protista are now placed in their own kingdoms, separate from Plantae. One consequence worth remembering is that cyanobacteria — the organisms popularly called blue-green algae — are not classified as true algae any more, even though the older, informal name persists in everyday use. With that boundary drawn, this chapter covers five groups that do sit inside Plantae: algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

Before going further, it is worth understanding why plants get grouped the way they do — the same question that comes up, for instance, when angiosperms are later split into dicots and monocots. The earliest classification schemes relied on easily seen, surface-level traits: growth habit, colour, and the number or shape of leaves, alongside floral characters. Carl Linnaeus's own system, for example, sorted flowering plants chiefly by the structure of the androecium (the male, pollen-producing part of the flower). Schemes built this way are called artificial classification systems. Their weakness is that they can split up species that are actually close relatives, because the grouping rests on only a handful of characters, and they treat vegetative features (roots, stems, leaves) and reproductive features as equally decisive — even though vegetative features are far more easily reshaped by the environment a plant grows in, so leaning on them equally is misleading.

Natural classification systems were developed to address this. Instead of a few visible traits, they draw on a plant's overall biological affinity with other plants — including internal, non-obvious features such as fine cellular structure, internal anatomy, how the embryo develops, and the plant's chemistry. The natural system that George Bentham and Joseph Dalton Hooker built for flowering plants is the standard historical example of this approach.

The classification systems used today go a step further still: phylogenetic classification, which groups organisms by their evolutionary relationships, on the working assumption that organisms placed in the same group share a common ancestor. To help resolve cases where the relationships are unclear — especially when there is no fossil evidence to fall back on — taxonomists now also draw on other, more specialised kinds of evidence. Numerical Taxonomy scores every observable character of an organism and assigns each one a number or code so the resulting data can be processed, today typically by computer, letting a very large number of characters all be weighed at once rather than a taxonomist favouring just a few by eye. Cytotaxonomy classifies using cell-level information such as chromosome number, structure and behaviour. Chemotaxonomy uses the chemical substances a plant produces to help settle cases that would otherwise stay ambiguous.