Fungus fools flies with fake flowers.

Normally a tall, Slender stem with delicate, pale blue flowers that droop off the top, this transformed rock cress sent up a yellow "floral " shoot because of a fungal infection. When a rust fungus invades this plant, called Arabis holboellii, the plant doubles the number of leaves produced and adds extra swirls to the rosette at its base. its stem then develops a dense cluster of yellow leaves that in one species of Arabis makes the plant look like a buttercup, both to insects and to botanists, says Barbara A. Roy, a plant ecologist at the University of California, Davis. The yellow color develops because male and female fungal sex organs cover the leaf surface.

At the Rocky Mountain Biological Laboratory at Gothic, Colo., she observed that these fake flowers attract flies, bees, and butterflies. These creatures tend to hang out at the yellow "pseudoflowers" up to five times as long as they spend at real flowers, Roy Says. That's because the fungus exudes a sugary fluid that makes the pseudoflowers smell sweet and can provide the insect visitor with 10 to 100 times the sugar of neighboring real flowers, she reports in the March 4 Nature.

That sweet stuff rewards the insect for its unknowing role in the life cycle of the fungus. As they visit the yellow "petals," these insects pick up and distribute sex cells and make possible sexual reproduction, Roy says. Then the yellow fades, and no more sweet fluid is produced. Other types of rust fungi make sweet fluids, but no other has been discovered that causes its host plant to make fake flowers.