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  Section: Introduction to Botany » Plant Classification
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Problems in Classification

Plant Classification
  Early Efforts at Plant Classification
  Carolus Linnaeus
  The Theory of Evolution
  Problems in Classification
  Monophyletic or Polyphyletic?
  System of Plant Classification
  What Is a Species?

These efforts have seen a measure of success; some parts of the scheme still, however, remain artificial. In fact, the five kingdom system employed in this text is yet on trial. The problems are not so acute with regard to higher plants such as the flowering plants. Here, flower parts are the most reliable indicators in determining allied forms. The number of parts, the way they are arranged, and the presence or absence of stamens serve to accurately classify flowering plants. The difficulty arises, rather, when working with microscopic forms. Even the distinction between plants and animals becomes hazy. It may be said, for instance,
that plants are distinguished from animals by the lack of contractile tissue or protoplasm; but this is not entirely true. It may also be said that plant cells have walls and animal cells do not. This is largely but not entirely true. It may be said that plants have the green pigment chlorophyll and, therefore, can carry on photosynthesis, whereas animals cannot. Some bacteria, however, are capable of photosynthesis. Using other microcharacteristics in an attempt to classify organisms is useful though also somewhat flawed. Chromosome number is an example. Some species have different chromosome numbers in different races. Xanthisma texanum has one less pair of chromosomes in the root than in the shoot. In the new field of chemical taxonomy, plants are grouped according to common chemical substances. The plants of the Cruciferae family, for example, all have mustard oils. Even the presence of common chemicals, however, does not guarantee kinship. Nicotine, for example, occurs in some widely separated families and not just in tobacco. Substances called tropolones occur in fungi (Penicillium) and also in certain conifers. The mention of these exceptions is not meant to entirely invalidate chemical taxonomy or other attempts at classifylng organisms. Rather, it is meant to illustrate the importance of considering a collection of information when attempting to identify relationships between organisms. The computer is being used as a tool to this end.

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