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Friday, January 06, 2006

Classifying Plants

Plant classification is one of the older pastimes in botany. Every culture, society, and religion has taken it as its duty to name and organize the plants in its area into some manageable arrangement. The arrangement at first was meant to facilitate communication between people about the plants under discussion.

The earliest organizations, naturally, were based upon the uses of plants by people. Obviously some plants were useful as vegetables, others were fruits, some make great spices for other foods, some were a good source of sugar, or perhaps starch. Oils could be extracted from others. Some were good for spinning fibers into thread for clothing. Yet others were ornamental.

But as is true of any classification scheme based upon human uses, there are problems of several sorts. The uses of plants by one civilization might not be the same as those by another. Even among individuals in the same society, one person's fruit is another's animal fodder. Moreover there is the conundrum of what to do with plants that present several different uses. George W. Carver demonstrated many uses for peanuts. Henry Ford produced an automobile with body parts made of soybean products. But the worst problem with human-use classification is that it puts related organisms in different classification categories. There is no relationship between the classification system and the evolutionary pathway through which the diversity was obtained.

Other early attempts at classification focused upon the form of the plant. Thus one category would be the herbs; here we use herb in its botanical sense meaning non-woody plants. A second category would be woody plants. These plants have secondary xylem and would need to be subdivided into several sub-categories of form: shrubs, vines, and trees. Each of these categories would have to be subdivided into deciduous and evergreen. Of course whether an evergreen vine is closer related to an evergreen tree or to a deciduous vine is a very good question. An of course there are both woody and herbaceous vines. It is sounding messy. Also, we find that a natural grouping have members in all of these "unnatural" categories.

Humans have also noticed that the seasonality of plants varies and forms another way to divide up the plants into categories. Thus in one category we have annual plants which are planted in the spring, flower that summer, and die in the fall. Then we have the perennial plants which are planted in one year, grow vegetatively in the first year, overwinter, and then flower in each year thereafter. Nurseries like to sell annuals because they know customers will come back next year to replace them all. They sell an entire flat of annuals for a small price. Perennials are planted, and then never replaced, so nurseries place a premium price upon them. However, there is a real "gotcha" among the perennials in some nurseries, however...biennials act like perennials in the first and second year, but die all the way to the ground at the end of that second year. You will have to replace those. Many seed catalogs are divided into annual and perennial categories. However even a casual gardener will be able to find plants of the same kind in both the annual and perennial categories. Certain species of a genus may be perennial, and other species of the same genus may be annual. It is not a natural breakdown.

Another way of dividing the plants is obvious in nursery catalogs: by climate. Tropical plants originate in areas of the world between the tropics of Cancer and Capricorn. This part of the world is very warm year-round with frost never occurring in the year. This is explained as the sun passes directly overhead on at least one day each year, and may do so for much of the year. Tropical plants generally cannot withstand any kind of frost. Subtropical plants are slightly more hardy and can tolerate light frost but not heavy freezes. Hardy plants can tolerate frozen periods of various durations and temperatures. The hardy plants are classified on the basis of hardiness zones. In North America these range from Zone 1 (extremely cold and long winters) to Zone 10 (no frost at all). Subtropical plants survive in zones 8-10. Some hardy plants can only survive in the warmer zones (7-10) while others are tough enough to survive in zones 1 and 2! In nursery catalogs the coldest zone for each plant is noted and gardeners are well-advised to heed those. Connecticut is zone 6 except for the northwest corner which is zone 5. Along the shore one might experiment with some zone 7 plants if a sheltered spot is available. However, again, close relatives can be tropical plants and extremely hardy plants. This is not a natural classification scheme. It is useful but not based upon genetics and evolution, the cornerstones of biology!


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