Angiosperm Classification The great diversity of angiosperms has made them especially difficult to classify in the scientific sense. For many years, flowering plants were classified according to the number of seed leaves, or cotyledons (kaht uh LEED uns), in their embryos. Those with one seed leaf were called monocots. Those with two seed leaves were called dicots. At one time, these two groups were considered classes within the angiosperm phylum, and all angiosperms were placed in one class or the other.

More recent studies of plant genomes and new fossil discoveries have shown that things are actually a little more complicated than that. For example, in 2002, an extraordinary plant fossil was discovered in northeastern China. Given the name Archaefructus, which means “ancient fruit,” this organism is the oldest known plant with reproductive organs like those found in modern flowers. It is more ancient than modern-day monocots and dicots and can't be classified as either.

Other recent evidence suggests that Amborella, a plant found only on the Pacific island of New Caledonia, belongs to still another ancient lineage of plants. Information gained from the Amborella discovery led scientists to place other plants, such as the water lilies, near the base of angiosperm evolution.

Figure 22–21 summarizes one modern view of angiosperm classification. Scientific classification now places the monocots into a single group but places the dicots in a variety of distinct and different categories. This means, of course, that the term dicot is no longer used for classification. However, it can still be used to describe many of the characteristics of plant structure, and that is how it is used in this book. DOL•30–DOL•33

FIGURE 22–20 Archaefructus

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FIGURE 22–21 Angiosperm Clades Five of the major clades of angiosperms are represented here. Scientists are still working out the relationships among these groups.

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