Current systems of classification

Division of organisms into kingdoms

As long as the only known plants were those that grew fixed in one place and all known animals moved about and took in food, the greater groups of organisms were obvious. Even in the time of Linnaeus, however, many biologists wondered about such animal groups as corals and sponges, which were fixed in position and in some ways even flowerlike. Were they zoophytes—animal-plants—intermediate between the two kingdoms?

A more serious problem of classification arose with the invention of the microscope and the discovery of microscopic forms of life. It became apparent that many of these microorganisms held both animal and plant characteristics and could not simply be classified in either kingdom. For example, Euglena is a unicellular organism with chlorophyll characteristic of a plant, yet with such animal features as an eyespot and locomotion by means of a flagellum.

Some microorganisms are parasitic inside animals and ingest complicated materials as food, while related microorganisms obtain their nutrients through photosynthesis. It has been proposed that the unicellular forms of microorganisms be placed in a separate kingdom, the Protista. Some biologists do not find this to be a happy solution, however, as some of the “unicellular” plants occur in “colonies” of various numbers of cells and may even have specialized reproductive cells.

In the mid-20th century, biologists recognized two vastly different cell types, procaryote (prokaryote) and eucaryote (eukaryote), and based a division of the living and extinct world on these two broad categorizations. The divisions were based primarily on the absence or presence, respectively, of a membrane-bound nucleus containing the genetic material of the cell, as well as on other organizational and structural features. Many classifications of living organisms adopted such a division and further created two superkingdoms, Prokaryota and Eukaryota. Within the Prokaryota was placed the kingdom Monera (the bacteria, blue-green algae, and a recently described bacterial group called the Archaebacteria [also called Archaeobacteria]). The Eukaryota comprised all other living organisms.

Viruses are far more difficult to classify. They are known only as parasites; no free-living forms have been found. They have a far simpler structure than bacteria and reproduce by injecting their hereditary material, which is either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) but not both (as in all other living things), into cells of other organisms. In effect, viruses utilize the host’s protein-synthesizing mechanism to reproduce. The individual virus particle (virion), therefore, does not grow and divide by fission as do bacteria. Some biologists have speculated that viruses are genes that have gotten out of control and become parasitic; others have denied that viruses can be considered living at all. Many are highly important disease producers in plants, animals, and bacteria.

The principal characteristic shared by bacteria and viruses is that the hereditary material is not contained within a special nuclear membrane. Such a prokaryotic condition might be postulated by evolutionists as primitive when compared with forms with a complex nucleus, as in eukaryotic organisms. Viruses as they now exist may be the simplest of living things, but it is not known how much they are modified from ancestral forms that are assumed not to have been parasitic and that were evidently on the main line of evolution; nor is their relation to bacteria known.

Another prokaryotic group, the blue-green algae, is traditionally placed with the other algae (e.g., seaweeds) and studied more by botanists than by microbiologists. Blue-green algae may be either unicellular or filamentous, and they behave like true plants, photosynthesizing in a way that resembles green plants rather than bacteria. Many move by gliding, as do some bacteria and some true unicellular algae. They are often extremely abundant around hot springs or at the edges of muddy ponds, and, though they are resistant to harsh environments, blue-green algae are killed by many drugs (e.g., antibiotics) used against bacteria. Perhaps they are best regarded as representing a group close to the main evolutionary line that gave rise to the eukaryotic plants.

Another problem relates to the position of the fungi, a large group including such familiar forms as mushrooms, toadstools, molds, and yeasts. (Although some authorities place the true slime molds [Myxomycetes] with the fungi, others point to the many characteristics they share with the protists.) The fungi are eukaryotic, lack chlorophyll (and therefore cannot photosynthesize as do green plants), and have rigid walls to the “cells,” or filaments (hyphae) that sometimes contain cellulose, as do green plants. Some fungi walls or filaments are made of chitin, the major constituent of the external skeleton of insects and other arthropods, or even of other structural compounds. A fungal “cell” usually contains many nuclei. Asexual and sexual spores are usually produced; some produce motile spores with flagella, like the spores of some algae. The sexual cycle is often very complex. Because fungi in general grow and produce “fruit” like ordinary plants, they have traditionally been included with them, but the differences between the fungi and the plants seem considerable.

The preceding considerations exemplify the difficulties inherent in producing a generally accepted classification, even at the highest levels. Since the earliest attempts at classifying the living world into two kingdoms, Plantae and Animalia, biologists have debated the relationships among all organisms. Most biologists, however, accept the fundamental differences in cell structure that separates the superkingdoms Eukaryota and Prokaryota.

The two-kingdom classification of organisms has not been a suitable alternative since the discovery of a microscopic group of organisms. One four-kingdom classification (Table) recognizes the kingdoms Virus, Monera, Plantae, and Animalia within the superkingdoms Prokaryota and Eukaryota. Separate kingdoms are not recognized for the microorganisms (Protista) or for the fungi, which are placed in the plant kingdom. Another classification recognizes Protista (including the fungi and protozoans) rather than viruses.