Trends of Specialization and Classification in Flowering Plants

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Various classifications of plants have been proposed ever since the 4th century B.C. when Theophrastus grouped plants into trees, shrubs, and herbs. The first classifications were merely for convenience and did not necessarily reflect natural relationships. Even today, plants may be lumped together in unnatural groupings in order to make them easier to identify. For example, some wildflower books arrange together all white-flowered species or all yellow-flowered species. There is nothing wrong with such identification arrangements, but because such schemes do not reflect natural relationships (i.e., relationships based on heredity and evolution), it is often difficult to recognize family characteristics or lineage. We don't infer that all persons with red hair are more closely related to each other than they are to those with dark hair or that all long-haired dogs are more closely related to each other than they are to short-haired dogs. Modern botanists, therefore, try to group plants according to their natural relationships. These are based on evidence gleaned from breeding experiments, form and structure, chemical components, fossil records, and other features. There are, however, many interpretations of trends in the specialization of flowering and other plants that are based primarily on inference from evidence presently available.

Although the information is very incomplete, the fossil record suggests that the flowering plants first appeared about 160 million years ago during the late Jurassic period (see Table 21.1), although the oldest known fossil of a flowering plant (discovered in China in 1998) has been dated at 142 million years. The flowering plants then developed during the Cretaceous period and the ensuing periods of the

Cenozoic era into the dominant elements of the flora they are today. Most botanists hypothesize that primitive flowering plants had their origins in the Jurassic period and had the following features: their leaves were simple; the flowers had numerous spirally arranged parts that were not fused to each other and were variable in number; the flowers were also radially symmetrical, or regular (i.e., the flowers could be divided into two equal halves along more than one lengthwise plane); and the flowers had both stamens and pistils.

Although today there still are many plants whose flowers have primitive features, various specializations and modifications have occurred since flowers first appeared. Flower parts have become fewer and definite in number. Some parts have fused, and spiral arrangements have been compressed to whorls (Fig. 23.9).

The first pistil is believed to have been formed from a leaflike structure with ovules along its margins. The edges of the blade, called a carpel, apparently rolled inward and fused together. In due course, the separate carpels of primitive flowers fused together, forming the common compound pistil consisting of several carpels, which is found in many of today's angiosperms (Fig. 23.10). Each segment of an orange, for example, represents a single carpel, and three carpels are easily distinguishable in a cross section of a cucumber.

In advanced flowers, the receptacle or other flower parts have fused to the ovary and grown up around it, so that the calyx and the corolla appear to be attached at the top of the ovary. When the ovary is embedded in the receptacle and other parts, it is said to be an inferior ovary, and the flower parts attached at the top of the ovary are said to be epigy-nous. A more primitive superior ovary is produced on top of the receptacle with the other flower parts attached around its base; such flower parts are said to be hypogynous. Some flowers have ovaries in an intermediate or half-inferior position, with perigynous flower parts, which are usually attached to a corolla tube of fused petals; the floral tube itself is generally not fused to the ovary (Fig. 23.11). Flowers have

A magnolia has numerous flower parts that are not fused together, superior ovaries, and radial symmetry

A magnolia has numerous flower parts that are not fused together, superior ovaries, and radial symmetry

Magnolia Flower Structure

stamens

Figure 23.Q Comparison between a primitive flower, magnolia (left) and an advanced flower, orchid (right).

pistils stamens inferior ovary pistils

Ovule Orchid Flower

anther cap stigma stamens and pistil fused together in a single structure anther cap stigma

An orchid has a reduced number of flower parts, fused parts, an inferior ovary, and bilateral symmetry.

Figure 23.Q Comparison between a primitive flower, magnolia (left) and an advanced flower, orchid (right).

Chapter 23

carpel carpel

Flower Ovary Cross Section

simple compound

Figure 23.10 Ovaries in cross section. A. The ovary of a simple pistil as found in a peach. B. The ovary of a compound pistil as found in a lily.

simple compound

Figure 23.10 Ovaries in cross section. A. The ovary of a simple pistil as found in a peach. B. The ovary of a compound pistil as found in a lily.

Simple Pistil

also tended to become bilaterally symmetrical, or irregular (i.e., capable of being divided into two symmetrical halves only by a single lengthwise plane passing through the axis), as in sweet peas and orchids.

If a flower has a calyx, corolla, stamens, and a pistil, it is complete. If, however, the corolla or other flower parts are missing, the flower is incomplete. If, as is the case with most flowers, both stamens and a pistil (or pistils) are present, it is said to be perfect, regardless of whether or not it also happens to be complete or incomplete. In some families, however, the flowers are not only incomplete but also have become imperfect (unisexual). Each imperfect flower has either stamens or a pistil but not both. The Pumpkin Family (Cucurbitaceae) includes pumpkins, squashes, watermelons, cantaloupes, cucumbers, and other species with imperfect flowers (Fig. 23.12). When both male and female imperfect flowers occur on the same plant (as is the case with many members of the Pumpkin Family), the species is monoecious. If a plant bears only male flowers and other plants of the same species bear only female flowers, the species is said to be dioecious.

In both the dicots and the monocots, evolutionary specialization has involved reduction and fusion of parts and a shifting of the ovary from a superior to an inferior position, as hypothetical progression is made from families with primitive characteristics through intermediates to families with advanced features.

Observations on the human and ecological relevance of the flowering plants occupy literally thousands of volumes. A brief overview of several aspects of this subject is given in Chapters 24 and 25.

inferior

(parts epigynous)

Figure 23.11 Ovary positions in flowers: superior (parts hypogynous; e.g. peach); half-inferior (parts perigynous; e.g., cherry); and inferior (parts epigynous; e.g., apple).

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Responses

  • Rebecca
    Is an superior ovary in flowers advanced or primitive?
    8 years ago
  • emiliano
    When plants were first grouped together it was merely for convenience?
    4 years ago

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