A giant flower's new family.

You may know someone who's taller, shorter, blonder, or more artistic than all of his or her relatives. The phenomenon can make you wonder at the strange ways in which family trees sometimes work.

Rafflesia plants present a similar puzzle. They boast the world's largest known flowers. With buds the size of basketballs and blooms that stretch 3 feet across, they can weigh up to 15 pounds. They're also among the stinkiest flowers in existence.

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For nearly 200 years, botanists have debated which plants are most closely related to rafflesias. Now, researchers from Harvard University have used DNA to put the plants in their place. Analyses of eight genes suggest that these megaflowers belong in the same family as poinsettias and castor beans.

The discovery is a surprise because poinsettias and many of their relatives in the Euphorbiaceae family have tiny flowers. (Poinsettias may look big and flowery, but their bulk actually comes from large, red, leaflike structures, not flowers.)

In fact, the species of Euphorbiaceae that are most closely related to rafflesias, according to the new study, have flowers that are just a tiny fraction of the size of rafflesia blooms. They measure just a few millimeters across. Some quirk in evolution during the last 46 million years led to the mega-boost in size, scientists say.

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Raffesias also evolved into parasites. They have no true roots or leaves. They live off of a plant in the grapevine family.

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Some experts are surprised by the new conclusions. The rafflesia plant's flowers seem too different in structure from those of poinsettias and castor beans to be related to them.

Even in the plant world, family trees can be confusing.--E. Sohn