Carnivorous plants are plants that obtains nutrients by trapping and digesting insects, other animals, or protozoa. Familiar examples of carnivorous plants are the Venus flytrap, pitcher plant, and sundew. The plants get energy from photosynthesis, but need nitrogen and other nutrients that they can’t get from poor soil. Carnivorous plants live all over the world, with the exception of Antarctica. They are angiosperms (flowering plants) and include both monocots and dicots.
Types of Carnivorous Plant Traps
In all cases, carnivorous plant traps are modified leaves. Plants use one or more of five trapping mechanisms:
- Snap trap: A snap trap catches prey using rapid leaf movement. Trigger hairs on modified leaves open ion channels when touched, snapping shut the leaf lobes. The lobes seal and form a stomach that digests the prey and absorbs its nutrients. The Venus flytrap (Dionaea muscipula) and waterwheel plant (Aldrovanda vesiculosa) use this trapping mechanism.
- Pitfall trap: A pitfall trap attracts prey with bright colors and nectar bribes. Prey lands on the slippery or waxy pitcher and falls inside. The pitcher water (Phytotelma) contains bacteria and digestive enzymes that break up the prey into a form the plant absorbs. Examples of carnivorous plants that use pitfall plants are Nepenthes (pitcher plant), Cephalotus (Albany pitcher plant), Serracenia (cobra lily), and some Bromeliads.
- Bladder trap: A bladder trap sucks in prey and digests it. The bladders pump out ions. Water follows the ions via osmosis, forming a partial vacuum. Aquatic invertebrates touch trigger hairs on the bladder, opening a pore, and sucking in the prey. Only Utricularia or bladderworts use this trap method.
- Flypaper trap: In a flypaper trap, leaves have coating of sticky mucilage that traps insects and other animals. In sundew plants (Drosera), the “glue” forms at the tips of tentacles that react to touch and wrap around prey. In butterworts (Pinguicula), the mucilage-secreting glands are short and the entire leaf surface is sticky. The leaf rolls around captured prey, forming a shallow pit where digestion occurs.
- Lobster pot trap: A lobster pot trap or eel trap captures prey using inward-pointing hairs that force prey toward the plant’s digestive region. Corkscrew plants (Genlisea) use this trap mechanism for catching and eating aquatic protozoa. Water movement through the trap helps force prey into an inescapable spiral.
Evolution of Carnivorous Plants
Charles Darwin published the first treatise on carnivorous plants (Insectivorous Plants) in 1875. Darwin believed carnivory was an example of convergent evolution. In other words, plants independently evolve and adopt similar strategies for meeting their nutrition needs. The ability to catch and digest prey has evolved independently at least 12 times. The earliest known carnivorous plant lived 85.6 million years ago. The newest is the bromeliad Broccinia reducta, which became carnivorous around 1.9 million years ago. At present, scientists recognize carnivory in five orders of flowering plants, which includes nearly 600 carnivorous species.
Sometimes the relationship between carnivorous plants and animals changes from predation into mutualism. For example, the Nepenthes pitcher plant has a mutually beneficial relationship with tree shews. The pitcher plant essentially fills “toilet pitchers” with exudates that are a food source for the shrews. The shrews mark and defecate into the pitchers, supplying the plant with nitrogen.
How to Grow Carnivorous Plants
Carnivorous plants are not particularly tricky to grow, but a lot of people kill them by treating them like typical houseplants. Like other plants, each species has its own requirements for light, temperature, and humidity. Unlike more familiar plants, carnivorous plants usually need poor, acidic soil and either rainwater, distilled, or reverse osmosis water. Tap water eventually kills many carnivorous plants because it contains calcium and other mineral that accumulate and become toxic. Many carnivorous plants need high humidity and damp soil, yet are susceptible to mold and parasites, especially if the temperature is too warm.
- Know the plant’s light and temperature requirements. Not all carnivores are tropical. Some need a period of cool temperatures or dormancy.
- Usually, keep humidity high. Placing the pot on a saucer of wet pebbles or growing in a terrarium addresses this need.
- Plant a carnivorous plant in poor, acidic soil. A 3:1 mix of Sphagnum peat or coir to sharp sand is a good general choice.
- Watch the plant for cues about its health. A plant getting proper light often becomes more colorful (usually red or purple).
- Outdoor plants feed themselves. Indoor plants accept careful hand-feeding. Don’t overfeed the plant with insects and never feed it meat or other inappropriate food. Plants rarely die from not-eating insects, but easily die from over-feeding.
- Barthlott, W.; Porembski, S.; Seine, R.; Theisen, T. (2007). The Curious World of Carnivorous Plants: A Comprehensive Guide to Their Biology and Cultivation. Translated by Ashdown M. Portland: Timber Press. ISBN 9780881927924.
- Clarke, C.M.; Bauer, U.; et al. (October 2009). “Tree shrew lavatories: a novel nitrogen sequestration strategy in a tropical pitcher plant”. Biology Letters. 5 (5): 632–5. doi:10.1098/rsbl.2009.0311
- Ellison, A.M. (November 2006). “Nutrient limitation and stoichiometry of carnivorous plants”. Plant Biology. Stuttgart, Germany. 8 (6): 740–7. doi:10.1055/s-2006-923956
- Hedrich, Rainer; Fukushima, Kenji (2021). “On the Origin of Carnivory: Molecular Physiology and Evolution of Plants on an Animal Diet”. Annual Review of Plant Biology. 72 (1): 133–153. doi:10.1146/annurev-arplant-080620-010429
- Slack, A. (1986). Insect-eating Plants and How to Grow Them. Sherborne UK: Alphabooks. ISBN 978-0-906670-42-2.