The tropical plant Genlisea is a tiny, homely rosette of simple green leaves. If you dig up its roots, you will find what look like an unremarkable bunch long, pale underground roots. Except they are not roots. They are death traps.
Insect-eating plants are famous the world round for their astounding animal-capturing and moving abilities, bright colors, and distinctive snares. The 450 carnivorous plants known to science often live in poor soils lacking in nitrogen, phosphorous and other minerals. Animals, it turns out, are excellent sources of these vital nutrients.
Genlisea is no different -- it lives in white sand and among damp rock outcrops in South America and tropical Africa. But it doesn't look like a stereotypical "carnivorous plant", nor does it capture insects. Instead, Genlisea and a small but growing number of other plants being discovered by scientists specialize in hunting prey that's much, much smaller.
Genlisea's "roots" are, in fact, highly modified leaves that lack chlorophyll or virtually any other clue that they were once leaves. The plant also lacks any true roots. Instead, these leaves perform the duty. They don't do this by sucking nutrients from the soil with the help of fungi like other upstanding plants. They do it by luring, trapping, and killing prey in traps along the special leaves.
Each modified leaf is solid on the end near the plant, but as it descends into the soil, it hollows out and then forks at a Y junction. The spiralling forks are just 200 micrometers wide inside, and they are covered by a series of traps. But whatever it is these traps have evolved to capture must be vanishingly small. The openings are slits a mere 400 micrometers wide by 180 micrometers long. At the opening of the traps are rows of hairs pointing toward the inside of the plant, as if to prevent the escape of anything so unfortunate as to find itself inside. There are also numerous glands.
Since the mid-19th century, people have suggested that this plant might be carnivorous and the suspicious slits traps. But insect remains were rarely found inside. Few insects could fit inside anyway. That left a surprising possibility: Genlisea sets traps to catch single-celled microorganisms called protists.
In tests with Genlisea plants and other plants from the same habitat, the ciliate Blepharisma americana -- chosen because it lives in roughly the same habitat as the plants and is an easy-to-spot shade of red -- was attracted to and entered the traps of Genlisea just minutes after the experiment began. They showed no similar curiosity toward plants from the same habitat. A chemical attractant is secreted from the traps, probably by way of the glands, and digestive enzymes also appear to be present. Radioactive sulfur ended up in the body of the plants two days after the plant had been exposed to radioactively-labeled ciliates. When the scientists looked inside the traps of wild Genlisea in the Ivory Coast of west Africa, nine different ciliates and various other protists were trapped "in large amounts".
But Genlisea, it turns out, is likely not the only plant that feeds on protists. And another that seems to do so is stranger still. That would be the delightfully quirky purple-worm liverwort, Pleurozia purpurea.
Liverworts already start out as funky and interesting plants. Along with mosses and hornworts, these plants attained their present form longer ago than any other land plants. They have retained many of the ancestral features they inherited from their green algal ancestors like swimming sperm, spores, and relatively simple bodies that most of the rest of land plants lost long ago.
But they added some new features as well: their reproductive structures can be flamboyant by the standards of any plant. The liverwort Marchantia makes three such structures -- mini female palm trees that produce eggs, male nails that make sperm, and splash-cups containing asexual reproductive tissue packets that get launched by wayward raindrops to greener pastures. Other liverworts' reproductive structures aren't quite so outlandish, but you get the idea. In addition, Pleurozia is unique among liverworts in its maroon color; most are a sedate green. So a red liverwort with a taste for prey seems like an especially delightful little package.
Pleurozia also makes traps, and the traps in question seem to have evolved from what amount to cisterns. Also found in several other liverwort lineages, these little sacs are normally used for storing water. They are usually lidless. But in Pleurozia and another genus called Colura, they have transparent, hinged lids. The lids of Pleurozia's traps open only inward, and are larger than the opening they block to prevent them from swinging outward. Once something slips past the one-way door, there is no escape. They look suspiciously like the insect traps of the flowering plant Utricularia, the world's fastest moving carnivorous plant:
But there is no way these traps could fit a crustacean like the one you saw in this video. The openings of Pleurozia's traps are a mere 300 micrometers wide. Most animals simply can't fit in a hole that size. So scientists suspected that this plant too might be after protists as well.
To test their hypothesis, the scientists who first explored this possibility used the same ciliate protist -- Blepharisma americana -- that the team who tested Genlisea used. When the scientists added a sprig of Pleurozia to their ciliate culture, most of the protists had moseyed on over to the plant to check it out.
86% of the water sacs contained ciliates 30 minutes later. One had caught 11. Several hours later, the sacs were stuffed with as many as 16 Blepharismas. Not a single ciliate escaped.
Unlike Genlisea, Pleurozia doesn't appear to have glands that secrete chemical attractants or digestive enzymes. The "bait" for these traps may be the natural community of bacteria that live on the surface of the plants. In other experiments, Blepharisma americana was observed feeding on bacteria by moving around plants "like a vacuum cleaner", and bacteria are frequently seen on the surfaces of liverworts in scanning electron micrographs.
What was perhaps most amazing about these experiments was the variety of creatures that showed up in the liverwort traps in plants from both in captivity and in the wild: turbellaria flatworms, rotifers, roundworms, flagellate protists, amoebae, copepod crustaceans, water bears, and mites. The sacs also often appeared full of schmutz, which the scientists interpreted as the decaying remains of the creatures caught in the traps.
Unfortunately, the scientists did not provide a control for their experiments as the Genlisea team did by placing other plants from the same habitat as Pleurozia in a petri dish with the microbes to see whether they proved as attractive. As well, and unlike the experiments with Genlisea, the scientists did not label their Blepharisma with traceable radioactive sulfur isotopes and therefore cannot be sure that the nutrients contained in the trapped unfortunates was actually taken up and used by Pleurozia.
So, they conclude, we can only call these plants zoophagous and not carnivorous because we are not 100% sure they are actually eating what they catch. The authors suggest that this can hardly be avoided, though, even if the plant produces no digestive enzymes. Little captured animals die and rot with the help of bacteria; deceased protists actually explode (due to the shut-down of their contractile vacuoles, which when running act like bilge pumps to expel water constantly taken aboard due to osmosis) and literally spill their guts into the trap. The nutrients are sitting there for the taking, even by passive pick-up. Still, all we have here is what prosecutors with thick southern drawls like to call strong circumstantial evidence.
While spotlight-hogging Venus Fly-Traps grab all the lurid glory, low-profile carnivorous plants like these two quietly trap protists and animals in tiny or underground chambers, and there are probably more species that do this yet undiscovered. But focusing on small game rather than large might offer an advantage: although the protist-traps of wild Genlisea and Pleurozia brim with victims, the insect snares of butterworts, sundews, and Venus fly-traps in the wild are only rarely observed to contain an actual fly. Perhaps it's not the size of the prey but the size of the catch that really matters.
Hess S., Frahm, J.,. & Inge Theisen (2005). Evidence of Zoophagy in a Second Liverwort Species, Pleurozia purpurea, The Bryologist, 108 (2) 212-218. DOI: http://dx.doi.org/10.1639/6
Barthlott W., Porembski S., Fischer E. & Gemmel B. (1998). First Protozoa-trapping Plant Found, Nature, 392 (447) 447. DOI: http://dx.doi.org/10.1038/33037