New Speedy Movements Seen in Carnivorous Plant

Biologists from the University of Bristol have discovered that a particular species of carnivorous plant needs its own category after observing the plant using movements never before seen in the plant kingdom. Writing in PNAS, the study authors suggest the Nepenthes gracilis is able to launch ants - its food source - to their death by using the power of falling raindrops. Carnivorous plants attract and consume insects to derive nutrients from them. Until very recently, there were two main categories of carnivorous plant. The first is the "active" category, where you would find various species of flytrap. These lure insects into the jaws of their "mouth" and snap closed suddenly until the plant's juices digest the insect. The second category is "passive", in which most pitcher plants fall. This is because the plants are motionless and rely on their slipperiness to slide their prey to their end. Pitcher plants are so named because that's what they look like - pitchers or jugs. The main body of the plant is the pitcher tube, which holds a sinister liquid inside that could easily be confused for sap. But this liquid is, in fact, the plant's digestive juices and the inevitable demise of any insect to fall into its depths. At the top of this tube is the peristome, the lip of the jug. This lip frames the mouth of the pitcher plant. Attached to the lip is a parasol-like lid called the operculum, the tip of which often hovers over the open mouth. And it's the lid of the Nepenthes gracilis that is particularly interesting. The team, led by Dr Ulrike Bauer, found that raindrops kick off very fast vibrations in the plant's operculum, which propel ants to the digestive juices waiting inside the pitcher tube. The study findings were based on the measurements of laser vibration caught on high-speed camera. The team was able to record extremely fast movements in the pitcher's lid-like leaf after it was hit with a raindrop. Dr Bauer explained that the movement is so quick because of its spring property: the operculum moves and then springs back. "You get an oscillation," she said, likening the movement to that of flicking a ruler on the edge of a desk. And intriguingly, the movement is unique in plants. Not only is it quicker than any other movements witnessed in carnivorous plants, but it also exploits an outside energy source to make this movement happen. As the team explained in their press release, all plants can move, but this movement is often too slow to be observed with the naked eye. "Sunflowers turn their heads to follow the path of the sun," the press release read. "Pot plants on a window sill turn their leaves towards the light." And while there are some fast-moving plants, such as those with catapult mechanisms to shoot seeds and the like or the flytraps we mentioned earlier, they need to use their own energy to power the movement. Apparently, the key to this fast movement lies in the stiffness of the pitcher's operculum lid. The team compared it to that of another species of pitcher plant. This second species catches ants using only the slippery rim of its pitcher tube and had a much more pliant lid than the Nepenthes gracilis. Bauer explained that the plant needs to maximise the area where insects fall from the surface of its lid, so the tension is spread throughout the whole operculum and not just the tip. She explained that otherwise the lid would act more like a diving board and wouldn't be an effective method of capturing food. The operculum is also coated with an anti-adhesive coating which makes it slippery enough that even the slightest instability in the leaf would cause the insect to lose grip and hurtle towards the digestive juices. And it's just the right amount of non-slip that when the leaf is calm, the ant can walk across with no worries at all. So it seems that the categorisation of carnivorous plants isn't as clear cut as just "active" and "passive". And the new "passive-dynamic" category has a solitary member in it - for now, at least. The study authors sum it up pretty well: "The passive-dynamic N. gracilis trap indicates the existence of a continuum between active and passive trapping mechanisms in carnivorous plants." Will there now follow mass re-categorisation? Watch this space!