I imagine that the first time a human saw a Venus flytrap in action, they were stunned. The first person was a Native American, but if they were a Southerner, they might have said, “I hope that thang don’t bite.” We call them carnivorous plants, but are they really carnivores? They don’t obtain energy from their prey, but they do get nutrients. It is more like the animals are multivitamins for the plant. They can obtain up to 50% of their nitrogen and phosphorus from captured animals. The bigger question is how did this evolve? We don’t know all the answers, but we do have some clues.
Supplementing nutrients with animals has evolved independently at least 12 times over the 140 million-plus years that flowering plants have existed. The oldest fossilized carnivorous plant is from 35 to 47 million years ago. It is similar to Roridula found in South Africa today. There have been at least six independent origins of pitfall traps, five of sticky traps, two of snap traps and one of lobster-pot trap. This bizarre strategy has repeatedly evolved in plants all over the world. Even more bizarre, they typically don’t capture pollinators. How do they know the difference? The flowers are spatially separated from the trap.
With pitfall traps, leaves are modified to create a pool that is often filled with liquid. The simplest pitfall trap is found in some bromeliads. Rainwater collects in leaves that form a cup. This is common with bromeliads, but a few species absorb nutrients from insects that get trapped in the water. Pitcher plants (Sarracenia) are a more complex pitfall trap, with individual leaves forming pitchers that may or may not be filled with water. Pitchers may have a hood to prevent excessive water collection. The plant often has nectaries (secreting a sweet substance) on the pitcher lip, produces odors and uses bright colors to lure unsuspecting prey to the slippery edge. The insect’s sledding trip down the pitcher quickly goes awry as it encounters downward-pointing hairs that prevent movement upward. It travels to the pit to drown or, if no water is present, starves. Enzymes secreted into the pitcher break down the insect, leaving only the exoskeleton. Voilà, another multivitamin obtained. I am glad I can’t fit in the trap.
A lobster trap is similar to a pitcher; however, it can be easily entered but the exit is difficult to find. Plants such as parrot pitcher plants (Sarracenia psittacina) have light-colored patches inside the hood that allow light to pass through. The insects try to exit through the lighter patches, end up bouncing off the pitcher wall and fall down the tube. Specialized cells called areoles allow the light to pass through. Once again, downward-pointing hairs force the prey to their doom at the bottom of the pitcher. How devious?
Plants such as butterworts (Pinguicula) have tiny glands on the upper surface of their yellowish basal rosette. The glandular secretions give the leaves a buttery or greasy feel. Pinguicula refers to “greasy one” in Latin. When small insects such as gnats, thrips or springtails get trapped on the leaves, their struggle causes more fluid and digestive enzymes to be released. The leaf curls into a cup shape as nutrients are absorbed, then flattens out. To prevent the insect from decaying before nutrients are extracted, the leaf releases an antibacterial substance. Don’t put this butter on your toast.
Sundews (Drosera) use a similar strategy. The leaves are long and threadlike or flattened like a spatula and covered with gland-tipped hairs that glisten in the sun. When an insect gets trapped in the sticky glands, the leaf begins to curl around the multivitamin. The more the insect struggles, the quicker the response. The leaf then secretes digestive enzymes and absorbs minerals. The deed is done.
Bladderworts (Utricularia) often live in open water and catch prey by suction. The prey is typically insect larvae or other tiny animals that swim or float. The bladder is like a suction bulb with a hairlike trigger. The hair senses movement and causes the bladder to suddenly fill with water and, hopefully, the prey. The motion is faster than the human eye can detect. Once inside the bladder, digestive enzymes break down the prey and nutrients are absorbed. Then the water is pumped out and the trap is reset.
The most highly evolved method of capture is the snap trap of the Venus flytrap (Dionaea muscipula). Prey enters a trap, brushes against trigger hairs and the trap quickly encloses it. Insects are attracted to the color, odor and nectaries of the modified leaf, which forms the trap. If an insect stimulates two or more trigger hairs in succession, the leaf quickly closes due to rapid changes in cell water pressure. The cells rapidly fill with water, causing them to expand and the trap to close. The more the insect struggles, the more tightly the trap closes. If there is no insect in the trap, it will reopen. The leaf trap secretes digestive enzymes that break down the fleshy portions of the insect. After about ten days, the trap reopens to release the exoskeleton and await the next meal. The trap can open and close seven times before it turns black and falls off. The trap essentially changes into a stomach by releasing digestive enzymes and then into an intestine to absorb nutrients. After it reopens, it becomes an insect attractant again. Talk about Jekyll and Hyde. They quickly go from beautiful and odiferous to digesting the unfortunate animal in a short period of time. During periods of water stress, the plant hormone abscisic acid causes trap closure to decline to reduce water use. For plants, water is a more important resource than nutrients.
All traps are modified leaves or parts of leaves, but the modified leaves sometimes act like roots by absorbing nutrients. All over the world, plants use the same genes found in roots to absorb nutrients, but they are activated in leaves instead. Here is another surprise: the traps use the same enzymes to digest prey and to protect themselves from bacteria, fungi and herbivorous insects. It is like they took their chemical weapons and started using them for digestion.
When you are walking through a bog observing these amazing carnivorous plants, just be grateful that “that thang don’t bite.”
Author
Robert Carter, Cooperative Extension, Area Forestry and Wildlife Agent
Clemson University Cooperative Extension Service offers its programs to people of all ages, regardless of race, color, religion, national origin, disability, political beliefs, sex, marital or family status and is an equal opportunity employer.
