Carnivorous plants are divided into two groups: plants with passive traps and plants with active traps. Pitcher plants have passive traps. This is the pitfall trap where an insect falls into the leaf and drowns in a pool of digestive enzymes. A carnivorous plant with an active trap moves a part of the plant to trap the insect. The Venus flytrap one such plant. The leaf blades fold closed, and stiff hairs around the edge of the leaf trap the insect.
The genlisea species grow in areas with nutrient-poor sand and moist rock outcrops in South America and Africa. The flowers are yellow or violet and grow on a stem that is about 2 feet high. The leaves form a rosette. When dug up, the plant has bundles of root-like organs that are not really roots. This plant is rootless---the organs live under the ground. The leaves have a hollow strand that merges into two twisted "branches," forming a "Y." Each twist contains a slit. Chemicals are released from the slits. The chemicals attract protozoa---the plant's food. They swim into the slit and are trapped, as they cannot swim out because of rows of hairs that point inward along the slits. The genlisea digests the protozoa.
The Venus flytrap belongs to the sundew family. It is native to North and South Carolina. When an insect lands in the leaf, the leaf closes over the insect. Hair along the edge of the leaves intersect, trapping the insect inside the leaf. The Venus flytrap produces digestive enzymes that break down the proteins of the insect, giving the plant much-needed nitrogen. Once the insect lands inside the leaf, it moves to get out and touches at least two of the three hairs inside the leaf. This action prompts the leaf to close over the insect.
The bladderwort (Utricularia) is the only plant with a true trap door. This aquatic plant has the fastest reaction to food of any carniverous plant. The branchlets have thousands of tiny bladders attached to them by stalks. When the bladders are flattened, they are about the size of a pinhead. One end contains an opening and a flap of tissue. The flap hangs down from the top of the opening, much like a garage door, except inward. Hair surrounds the opening. The tiny glands inside the bladder absorb water, then expel it outside, which causes a slight vacuum. When an aquatic organism touches the hairs, the watertight seal breaks from around the door, and the vacuum sucks the organism inside, where enzymes digest the organism and provide the plant with nutrients.