Scientists have solved an underground mystery: Why does a plant that survives on sunlight grow leaves beneath the earth?

Flowering plants of the genus Philcoxia are the only known plants with the "awkward" feature of subterranean leaves, said Rafael Oliveira, a plant biologist at the State University of Campinas in Brazil.

Oliveira's new research sheds new light on the oddity, showing that the leaves act as traps for tiny roundworms, or nematodes. This worm food is vital for the plant's survival in the nutrient-deprived savannas of central Brazil.

Plants may seem "boring for some people, because they don't move or actively hunt for their food," Oliveira said by email.

But "they have evolved a number of fascinating solutions to solve common problems, such as the lack of readily available nutrients or water."

Feeding Worms to Plants

Oliveira and colleagues had suspected that Philcoxia plants may be carnivorous, because their sandy habitats and their physical features—such as poorly developed root systems—resemble those of known carnivorous plants. The team had also recently observed roundworms on the plants' subterranean leaves.

To test their hypothesis, the researchers bred nematodes in nitrogen, a marker that would allow the scientists to know if the plant indeed digests worms.

The team then "fed" the nematodes to plants in the lab, and harvested their leaves 24 and 48 days later. A chemical analysis revealed nitrogen from the worms had been incorporated into the plant's leaves.

The results add up to the first evidence of a carnivorous plant with specific adaptations for trapping and eating roundworms, he added.

More Killer Plants Out There?

Only 0.2 percent of flowering plant species are known to digest meat. (See pictures of killer plants.)

But "if we start to look closer at microorganisms [such as nematodes] as a prey type," Oliveira predicted, "we might find more carnivorous plants."

Many plant behaviors, he added, "operate hidden from our view."