Original Cell

Figure 16.4 Hypothetical derivations and relationships among kingdoms and the major groups of organisms.

Chapter 16

The second part of the scientific name of a species, called the specific epithet, is followed by the name of the author, mostly in abbreviated form. The author is the person (or persons) who originally named the plant or placed the species in a particular genus. Many plants have one author's name in parentheses, followed by the name of another author outside the parentheses. For example, the scientific name of mountain hemlock is Tsuga mertensiana (Bong.) Carr. In this instance, August Bongard (abbreviated to Bong.), a professor of botany at St. Petersburg in the early 19th century, first described the species under the genus Pinus. Half a century later, Êlie Carrière (abbreviated to Carr.), an authority on conifers at the Paris Natural History Museum, transferred the species to the genus Tsuga. Because Bongard was the first to describe the species and Carrière was the first to place the species in a different genus, both names are included with the scientific name. Binomials are printed in italics (or the words are underlined). In the case of Allium cepa, the L. following the name stands for Linnaeus, since this plant was one of the 7,300 species he included in his Species Plantarum.

In addition to these categories of classification, various "in-between" categories, such as subphylum, subclass, and suborder, have been used, and species themselves are sometimes further divided into subspecies, varieties, and forms. A few authors also recognize "super" categories, such as superkingdom and superorder. One classification widely used in the past recognized only two divisions (phyla) in Kingdom Plantae (Division [Phylum] Bryophyta for nonvascular plants and Division [Phylum] Tracheophyta, with a number of subdivisions [subphyla], for all vascular plants). These subphyla are treated as full phyla in this and a number of other texts.

Since human judgment has to enter into the compilation of classifications, there undoubtedly will never be complete agreement on them, but they are nevertheless useful, indeed necessary, for making some order of the diversity of life around us.

Those who specialize in identifying, naming, and classifying organisms are called taxonomists. Today, taxono-

mists who involve evolutionary processes in trying to sort out natural relationships refer to themselves as systematists. One of the activities of taxonomists and systematists is the construction of keys to help others identify organisms with which they may not be familiar. Most such keys are dichoto-mous; that is, they give the reader pairs of statements based on features of the organisms. By carefully examining an organism and choosing from each pair the statements that most closely apply to the organism, we can arrive at an identification. If, for example, we agree with the first of the two statements given for each number in the key, then we either use the identification at the end of the line or proceed to the next indented statement beneath. If we disagree with the first of the paired statements, then we look for the matching alternate statement and proceed from there.

The key that follows this discussion deals with major groups of organisms and illustrates natural relationships. It covers only general features and does not indicate various details or occasional exceptions that would appear in keys for lesser groups. All organisms in any given group are presumed to be more closely related to each other than to organisms in another group.

Once again, it must be emphasized that a key of this type, while calling attention to basic differences between major groups of organisms, may also oversimplify the differences, partly because it does not list all the characteristics of each member of a given group. Moreover, it does not necessarily identify exceptions to the rule or some of the intermediate forms that frequently are transferred back and forth as research uncovers new evidence or as new interpretations develop.

Keys at this level of classification also are not completely practical because to be able to arrive at an identification, one sometimes has to have available specific stages in the life cycles of the plants. Furthermore, chemical tests that are not easily performed or details that may be difficult to see with a light microscope may be required. Nevertheless, such a key is generally preferable to a simple listing of groups and their characteristics for purposes of distinguishing among them.

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Essentials of Human Physiology

Essentials of Human Physiology

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