The rim around the opening to a pitcher plant’s trap, the peristome, is distinctive. Often ridges run down into the pitfall trap, but why? Why not have something barbed instead to hinder insects climbing back out? David Labonte and colleagues have examined the topography of these rims. They conclude the entrances fold in an intricate way to increase the wettability and slipperiness of the traps.

Nepenthes is a genus of plants that hunts in the tropics, mainly in southeast Asia. It produces modified leaves that form cups, or pitchers, with a lid over the top. Prey, usually arthropods, comes along to feed at the pitcher and, while venturing on the rim, falls in. Some plants do this by producing wax crystals that can stick to insect pads and prevent grip. But not all Nepenthes plants do this. Some use water to create a wet trap. How the plant can use water reliably, without it running away rapidly, is a puzzle.

The wet trap works by using water to create a low grip surface on the plant. The plant does this by holding a thin film of water in place so, ideally, you’d have a very hydrophilic surface. The problem with that is that the cuticle, the outermost layer of the plant, needs to be a barrier to water to keep moisture in when the weather is warm. These conflicting needs, to be very wet yet remain a waterproof barrier, are why Labonte and colleagues investigated the traps.

The answer lies in the ridges on the peristome, which come in two forms.

A pitcher plant’s peristome. Image: Canva.

The easiest ridges to see are the macroscopic ridges. These form polar channels, running up and down into the trap. Water falls into these ridges and runs down, wetting them. What water doesn’t do very well is run across the ridges. The team experimented to see how difficult it was to for water to travel across from one major channel to the other. The answer is very.

What they found is that water would pile up in a channel. Once the water was deeper than the channel, the water would build up as a bulge instead of overflowing until it reached a critical angle. Then a mass of it would move to fill the next channel. Instead of gently flowing across, it moved in a series of haphazard steps. Combine this with the ease water is guided to run down the channels, and it’s clear that once water enters the peristome, it runs in a very specific direction instead of meandering across.