If you were to ask the average person the differences between plants and animals, you would probably be told that plants are green, don't eat each other, and don't move. Animals, on the other hand, are not green, do eat plants or each other, and do move. A distinction between plants and animals in general has probably always existed in the minds of intelligent beings, and it was natural when classification schemes were first developed that all living organisms would be placed, according to the highest category of kingdom, in either the Plant Kingdom or the Animal Kingdom.
While this distinction still works well for the more complex plants and animals, it breaks down for some of the so-called simpler organisms. For example, there are more than 300 species of single-celled organisms called euglenoids (see Fig. 18.23) inhabiting a variety of freshwater habitats. These microscopic creatures have small, whiplike tails called flagella that pull them through the water, and they also can ingest food particles through a groove called a gullet. Both of these features would be considered animal-like, and so euglenoids in the past have been treated as animals in some textbooks. Many of these organisms also, however, have chloroplasts, and if light is present, they can carry on photosynthesis efficiently enough to eliminate the need for ingesting food. Accordingly, they have often been treated as plants in botany books.
A similar problem has existed with slime molds, which resemble masses of protoplasm slowly creeping or flowing across dead leaves and debris, engulfing and feeding on bacteria and other substances as they go, and looking somewhat like amoebae, which botanists and zoologists alike have traditionally called animals. When slime molds reproduce, however, dramatic changes take place. They become stationary and develop reproductive bodies that are distinctly fungus-like, causing some to consider them fungi.
In an attempt to overcome this problem, the biologists J. Hogg and Ernst Haeckel proposed a third kingdom in the 1860s. All organisms that did not develop complex tissues (e.g., algae, fungi, and sponges) were placed in a third kingdom called Protoctista. This third kingdom included such a heterogeneous variety of organisms, however, that in 1938, another biologist by the name of Herbert F. Copeland pro posed that it again be divided. He assigned the name Monera to all single-celled organisms with prokaryotic cells (the differences between prokaryotic and eukaryotic cells are discussed in Chapters 3 and 17), leaving the algae, fungi, and single-celled organisms with eukaryotic cells in the Kingdom Protoctista.
Although many biologists thought Copeland's four-kingdom system of classification was a definite improvement, it too had problems—particularly the fact that basic differences in the mode of nutrition existed among organisms within the Kingdom Protoctista. As a result, during the 1970s, many biologists adopted variations of a five-kingdom system proposed by R. H. Whittaker in 1969 (Table 16.1).
In Whittaker's system, three kingdoms of more complex organisms based on three basic forms of nutrition (photosynthesis, ingestion of food, and absorption of food in solution) were recognized, along with two kingdoms of pro-tists, which were distinguished on the basis of differences in cellular structure.
The five-kingdom arrangement also has had critics and problems, particularly since the early 1980s when Carl Woese, a microbiologist and leading authority on bacteria, argued cogently that Kingdom Monera should be divided into two kingdoms. Woese based his contention on some previously unrecognized fundamental differences between two major groups of bacteria (discussed in Chapter 17). His proposal for two kingdoms of bacteria gained increasing acceptance in the 1990s and is now generally recognized.
Since acceptance of Woese's kingdom classification, James Lake and his colleagues have proposed creating yet another kingdom by dividing Kingdom Archaea into two kingdoms, making three kingdoms of prokaryotes in all. The slime molds, which have no cell walls during their active state but do develop walls when they reproduce, still do not fit well into the six-kingdom system, and other organisms traditionally regarded as fungi may be more closely related to protists. Much information still needs to be accumulated to understand natural relationships, and until this occurs, taxonomic categories will remain subject to different interpretations.
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