The classification of plants

Any classification system involves the grouping of organisms or objects using characteristics common to members within the group. A classification can be as simple as dividing things by colour or size. Fundamental to most systems and making the effort worthwhile is that the classification meets a purpose; has a use. This is generally to make lifesimpler such as to find books in a library; they can be classified in different, but helpful ways, e.g. by subject or date or particular use.

Terms that are used in classification are:

• taxonomy, which deals with the principles on which a classification is based;
• systematics identifies the groups to be used in the classification;
• nomenclature deals with naming.

Various systems for organisms have been devised throughout history, but a seventeenth century Swedish botanist, Linnaeus, laid the basis for much subsequent work in the classification of plants, animals (and also minerals). The original divisions of the plant kingdom were the main groupings of organisms according to their place in evolutionary history. Simple single-celled organisms from aquatic environments evolved to more complex descendants, multicellular plants with diverse structures, which were able to survive in a terrestrial habitat, and develop sophisticated reproduction mechanisms.

The world of living organisms is currently divided into five kingdoms including:

• Plantae (plants)
• Animalia (animals)
• Fungi
• Bacteria (Prokaryotae)
• Protoctista (all other organisms that are not in the other kingdoms including algae and protozoa).

The organisms constituting the plant kingdom are distinguishable from animals in having sedentary growth, cellulose cell walls and polyploidy. They are able to change energy from one form (e.g. light) into organic molecules (autotrophic nutrition; see photosynthesis). Animals, amongst other things, have no cell walls and rely on eating ready-made organic molecules (heterotrophic nutrition).

Plant divisions (the names ending in - phyta) are further sub-divided into

• class (ending -psida);
• order (ending -ales);
• family (ending -aceae);
• genus;
• species

Species is the basic unit of classification, and is defined as a group of individuals with the greatest mutual resemblance, which are able to breed amongst themselves. A number of species with basic similarities constitute a genus (plural genera), a number of genera constitute a family, and a number of families make up an order (see example given in Table 4.1). Subspecies are a naturally occurring variation within a species where the types are quite different from each other.

In order to produce a universally acceptable system, the International Code of Botanical Nomenclature was formulated, which includes both non-cultivated plants and details specific to cultivated plants.