Just 0.2 percent of the flowering plants in the world are known to be carnivorous. We’re most familiar with Venus Flytraps, pitcher plants, and other plants that capture and digest their prey with showy techniques. However, there are other carnivorous plants that are much sneakier in their murderous ways. This week, PNAS reports that a plant with a previously unknown method of carnivory has been discovered; it catches and consumes its prey underground.

Philcoxia minensis belongs to a small genus of plants that grow in the Cerrado region of Brazil. Like many other carnivorous plants, it lives in a bright, moist, low-nutrient environment and has a nonmycorrhizal root system, meaning that it doesn't form a symbiotic relationship with fungi to help it obtain nutrients. That led researchers to suspect that it might get those nutrients through carnivory. However, its method of prey capture wasn't obvious at first glance because from the surface, there’s no sign of any type of trap.

It turns out that P. minensis uses sticky, subterranean leaves to capture nematodes, small worms that live under ground. Below the surface of the sand, the plant grows a set of tiny leaves that secrete an adhesive substance to trap the worms. Once the prey is stuck, the plant can go ahead with the business of absorbing nutrients from the worm’s body.

Carnivorous plants can get nutrition from their prey in two ways: either directly, via high enzyme activity on their leaves, or indirectly, by having microbes break the prey down (as pitcher plants do). P. minensis takes the direct route, digesting the worms itself. To test the efficiency of this process, the researchers injected nematodes with a nitrogen isotope, then fed them to P. minensis. Within 24 hours, 5 percent of the labeled nitrogen was present in the leaves, and after 48 hours, 15 percent was found in the leaves. With uptake levels this high, P. minensis seems to be more efficient at nutrient acquisition than many other carnivorous plants.

This is the first conclusive evidence of carnivory in the very diverse Plantaginaceae family. The other two species in the genus Philcoxia also live in similarly poor conditions, and have many of the same morphological features as P. minensis, so it’s likely that they are carnivorous as well. With this type of cryptic carnivory, it’s likely that more plants may capture and consume invertebrate prey than we currently realize.

PNAS, 2012. DOI: 10.1073/pnas.1114199109 (About DOIs).