Botany/Kinds of plants

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Chapter 7. Plant Systematics

The Kinds of Plants
The total number of species of plants is tremendous. Any attempt at representing their great diversity requires a system of ordering or arranging the many plant types&mdash;hopefully a system that itself contributes to our knowledge and understanding. We could take the straightforward approach of listing all plants alphabetically by their common name, or perhaps by their species name. This index approach would be handy, but would not tell us much about the plants themselves. We would have to know the plants whose names are included for the list itself to have any meaning to us. However, botanists developed a hierarchical system based on the fact&mdash;considering a variety of characteristics&mdash;that some plants are clearly more similar to each other than to other plants. Arranging all of the world's plants (and animals and minerals) by similarities to each other was an idea first promoted by Carolus Linnaeus.


 * Reread Plants ( Links need not be perused )
 * Read Linnaean taxonomy ( follow links )

Categorizing is an important process by which we humans gain understanding of the world around us, and something we all do to some degree as part of our observation of things and events that we encounter. In biology, as the concept of evolution was formulated, it became obvious that this concept could be the basis for categorization. If plants that are similar in form are indeed closely related&mdash;at least more closely related than plants that are dissimilar in form&mdash;then a system of classification could be devised that reflected these relationships. This approach has important implications. Related plants have common properties, a fact that can be exploited in agriculture and other practical botanical fields.

Initially, botanists had but one approach: physical examination. The careful examination (and detailed description) of plant structures allowed for arranging each species within a system that placed all more or less similar plants (in certain "important" features) together. This approach is not as easy as it sounds, but played off of and contributed to the expansion of descriptive botany in the 18th and 19th centuries. One problem that became evident is that as species evolved, unrelated plants could come to resemble each other in many respects. After all, form and function are closely related. Within similar habitats (say deserts), species of very distantly related plants might well evolve towards a similar form. Species do not have (or certainly did not have over geological time) unrestricted access to all places on the planet and species distribution is then an important part of interpreting the evolutionary process. As species evolved, they did so within the constraints to dispersion that existed at the time. This fact provides an important clue: unrelated but similar plants are likely to be distributed far from each other on the Earth's surface; and the corollary: plants that have similar structures but have widely separated distributions, may not be so closely related in an evolutionary sense.

At this point it is worthwhile to consider some examples. There are many succulent plants, as this form (typically thick, fleshy stems and/or leaves; often reduction or complete loss of leaves) incorporates adaptations necessary for a plant to survive very dry conditions. Non-botanists are tempted to classify all such plants as types of cacti. In fact, cacti evolved in the New World (the Americas), yet there are many succulents (and many plants that resemble cacti) that are not native to the New World, and evolved independently on the African continent. A large group of such plants are known as the euphorbs.