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What is plant taxonomy?
Taxonomy is about grouping things; plant taxonomy is about grouping plants.
There are many ways to group plants. Many popular flower books group species by color; gardeners may group them by a combination of habit and color; herbalists might group them by their effects on humans. These are all valid special purpose groupings, but plant taxonomy is about grouping plants in such a way that the groups have predictive value. This means that, if you know a plant belongs to group A rather than group B, it will also share other characteristics of group A, even those that were not used to decide which plants go into group A. It is this predictive ability that makes the groups of scientific plant taxonomy particularly valuable.
Predictions based on a taxonomic group are not always correct, but they are more likely to be correct than incorrect if the taxonomic group is appropriately circumscribed. Consider some examples:
Example 1: You are told that a plant is a tree. Can you predict whether it has flowers or cones? Suppose you are told, in addition, that it has blue flowers. Can you tell whether the flowers are radially symmetric (circular in outline when viewed face-on) or bilaterally symmetric (two-sided in outline when viewed faced-on)?
Example 2: You are told that a plant is a member of the Brassicaceae (a family of plants that includes mustards and cabbages). By the end of this course, you will be able to state with considerable confidence that it probably has simple or pinnately dissected alternate leaves and flowers with four separate petals, 6 stamens (2 of which are shorter than the others), a superior ovary with (very probably) 2 chambers, and ovules that are attached to the walls of the ovary. If you really dig into the information that has been accumulated about the family, you will find that you can also predict that the plant manufactures glucosinolates but not iridoids. What is particularly pertinent to this discussion is that taxonomists had agreed on the circumscription of the Brassicaceae before anyone knew anything about glucosinolates and iridoids.
How does one decide what makes a good taxonomic group?
Obviously, in comparing the characteristics of different plants one needs to be comparing homologous structures (structures that are the product of the same genes). One cannot, for instance, compare the shape of a leaf in one plant with the shape of a petal in another plant. This seems fairly obvious, but the leaves of some plants look very like petals.
In recent years, taxonomists have started considering several non-morphological characteristics in forming plant groups. Among such characteristics are ecology, chromosome numbers, isozymes (proteins produced by particular genes), and nucleic acids sequences. They have also started using explicit means of numerical data analysis to evaluate the relationships among different organisms. We shall discuss these other characteristics and the two primary methods of numerical data analysis briefly in this course, but our primary focus is going to be on learning to use groups that have already been circumscribed, not on reevaluating such groups.
Names of the groups used by taxonomists
Kingdom, Division (may be called phylum), Class, Order, Family, Tribe, Genus, Species, Variety, Form
Every plant belongs to a species, every species to a genus, every genus to a family, every family to an order, every order to a class, and every class to a division, but species do not have to have varieties or forms.
If additional ranks are needed, one can use the prefixes 'super-' or 'sub-' to inject ranks above or below, respectively each of the main ranks. For instance, superfamilies contain two or more families but a subfamily would refer to a part of a family. In this course we shall sometimes refer to subfamilies and subspecies, but we are not likely to use any other prefixed ranks.
There are times when we want to talk about taxonomic groups without being explicit about the rank involved. The word taxon (plural taxa) was invented for this purpose. It may be that we simply wish to avoid a discussion as to whether the group should be treated as a species or a subspecies or it may be that what is being said is equally true about species, genera, families, and divisions.