Plant classification

Kingdom Plantae

Division Magnoliophyta

Class Magnoliopsida

Subclass Rosidae

Order Rosales

Family Rosaceae

Genus Rosa

Species arvensis

Scientific classification is the system most commonly used in describing plants. Using this method, a common field rose could first be grouped into those organisms that are eukaryotes— that is, they have nuclei in their cells. This group embraces most animals and plants and is considered to be a kingdom by some botanists. The grouping can then be broken down in stages to the basic unit—the plant’s species name, Rosa arvensis.

The classification of plants and animals provides a logical system by which living organisms can be accurately identified. But in its widest sense, plant classification means more than just giving a name to each of the more than 400,000 or so species of which the plant kingdom is composed. It consists of grouping plants in categories of ever-increasing size—a process known as scientific classification. If a classification also takes account of the relationships that exist between plants then it is described as systematic. The entire process of classifying organisms, and the study of classifications past and present, is embraced by the blanket term taxonomy.

The foundations of modern taxonomy were laid in the mid-eighteenth century with the work of the Swedish naturalist Carl von Linne (1707-1778), better known by the Latinized version of his name—Linnaeus. It was Linnaeus who, with the publication of his Species Plan-tarum in 1753, established the system of binomial nomenclature for plants—such as Rosa arvensis for the field rose.

Naming and grouping

Today the naming of plants and their placement in a standard, accepted hierarchical system of classification is governed by strict rules laid down in The International Code of Botanical Nomenclature (ICBN). This rule book recognizes 12 principal ranks of diminishing size, ranging from kingdom through phylum and species to form, with subcategories that can be designated for each category. Of these, the basic “unit” of classification is generally considered to be the species.

A species is a group of individuals that are so similar as to be more or less identical and that can interbreed. Similar species are grouped into genera, similar genera into families, and so on.

Despite the fact that plants are grouped and named in such a rigid way, they are not classified using just one system. Scientific classification is an interpretation of facts. It is based on the opinion and judgment a biologist forms after studying specimens of animals and plants. Most biologists use the same basic framework for classification. But not all biologists agree on how individual animals and plants fit into this scheme, and so classifications often differ in details. The system used here differs slightly from that discussed in The World Book Encyclopedia. For more information on scientific classification, seethe World Book articles CLASSIFICATION, SCIENTIFIC and PLANT (A classification of the plant kingdom).

Scanning electron micrographs are often used to study pollen grains and spores. Such microscopic examination of these bodies reveals their structural characteristics, which can be used as a means of grouping together species of plants.

Any system of classification that takes into account only one or a few plant (or animal) characteristics is described as artificial. Such an artificial system could group plants, for example, according to their color, petal number, or edibility. While they are useful for specific purposes, such systems do not have great worth in the wider botanical context. By comparison natural systems, based on the analysis of a great many characteristics shared (or not shared) by plants, are very useful.

In some aspects of botanical study, however, demands are made that are not necessarily met by a natural system of classification; notable among these is the study of the evolutionary relationships between plants.

This system is designed to reflect in particular the evolutionary pathways along which plants have progressed over millions of years of development.

Dendrograms are a graphic means of showing the relationships between groups of plants. A pheno-gram (left) charts the percentage of phenotypic similarities between a number of taxonomic groups. A cladogram (right) indicates the evolutionary pathways of species overthe past millions of years to reveal their lineage relationships.

Gathering information

Any system of classification relies on the accumulation of vast amounts of data about plants. This information includes the gross structure of a plant (its morphology), often with special reference to flowers and other organs involved in reproduction. Plants are measured and examined microscopically in every detail with light and electron microscopes. The development of each plant is studied macro- and microscopically, and its number of chromosomes, breeding systems, geographical distribution, and ecology are all considered. Chemistry and DNA sequencing can be significant, and the fossil record can also provide valuable insight into the way in which plants should be grouped together.

Fossils of plant organs such as leaves are an important source of material for the evolutionary classification of plants.

The practice of numerical taxonomy is based on the construction of similarity tables or data matrices between species, genera, and other classifications. It relies on computers to store and analyze information and to calculate the degrees of similarity between plants. By this means, plants are evaluated and grouped according to the degree of similarity they possess.

Plant classification is a means by which we can keep track of the changing nature of the plant kingdom, for example by monitoring plants that are in danger of extinction, particularly now that many of our traditional natural resources are in short supply or running out. At the laboratory level, experimental taxonomy, which is designed to discover more about the nature and genetic basis of the diversity of species, could hold the key to the preservation of that diversity and its use in such tasks as the improvement of crop plants.

Illustrated herbals played a significant role in the early description and subsequent classification of plants.
Many portrayed the whole plant and its individual parts.