An insectivorous plant, also called a carnivorous plant, captures prey items, such as insects, spiders, crustaceans, mites, and protozoans, as a nitrogen source. Many insectivorous species live in freshwater bogs, where nitrogen is not present in available form, because the pH of the water is extremely acid.
The forms of entrapment by these types of plants are modified leaves.
General Taxonomy of Insectivorous Plants.
The insectivorous plants are found in nine angiospermous families.
- Bromeliaceae (Brocchinia hectioides, B. reducta, Catopsis berteroniana of the Neotropics)
- Byblidaceae (Byblis gigantea, B. liniflora of Australia and New Guinea)
- Cephalotaceae (one species, Cephalotus follicularis, endemic to Australia)
- Dioncophyllaceae (Triphyophyllum peltatum of western Africa)
- Droseraceae (about 110 species; Aldrovanda, Dionaea, Drosera, Drosophyllum)
- Lentibulariaceae (Genlisea, Pinguicula, Utricularia)
- Nepenthaceae (Nepenthes, 70 spp. of Southeast Asia and Madagascar)
- Sarraceniaceae (Darlingtonia, Heliamphora, Sarracenia)
Judging from the current placement of these families within a phylogenetic classification of dicotyledons, the insectivorous habit appears to have evolved independently numerous times.
The General Types and Properties of Insectivorous Plants.
- Active traps: involves rapid plant movement
- Closing traps.
- Example: the Venus' flytrap, Dionaea muscipula. The leaf blade is bivalved, convex, but not hinged per se, and bears up to six sensitive hairs per half leaf. An animal must trip two or three sensitive trigger hairs (trichomes), which causes the bulge to be reversed suddenly, closing the leaf. Also on the upper (adaxial) leaf surface are multicellular hairs that secrete digestive enzymes. Digestion of an insect requires three to five days. If no prey was caught, the trap will open within an hour. Aldrovanda also has an active trap.
- Example: bladderworts, Utricularia spp. This worldwide genus of approximately 180 species inhabits ponds and lakes. The bladder, a modification of an underwater leaf segment, is a one-way door. Fluids are absorbed during the resting stage to fill the bladder with air. There are sensitive hairs located at the edge of the trapdoor, and when stimulated they cause the valve to move due to a sudden change in electrical potential. When the trapdoor swings inward, there is a sudden inrush of water (1/460th of a second), pulling in the aquatic organism, and then the door swings into the closed position. Digestive enzymes are secreted by the surrounding tissue, and prey are digested within several days. If prey are not captured, the trap resets within 30 minutes. When a bladderwort is lifted from the water, on which it floats, there is a fine crackling sound as the trapdoors are triggered.
- Passive traps: involves no rapid plant movement
- Pitfall traps.
- Example: pitcher plants, Sarracenia spp., Darlingtonia californica, Heliamphora spp., Nepenthes spp., bromeliads, and Cephalotus follicularis. Prey are lured and deceived to enter the pitcher, a cylindrical, modified leaf, and then prey cannot escape, usually due to downward-pointing hairs (trichomes). In the pitcher of Sarracenia, the lid prevents dilution of the liquid contents, which contains digestive enzymes (glands in the lower pitcher), and plant wax impedes footings on the upper half of the pitcher, which causes victims to fall into the liquid below. In the pitcher of Darlingtonia, there are no digestive glands, and only microorganisms are consumed.
- Flypaper or adhesive traps.
- Example: sundews, Drosera. Coloration and sweet nectar from glands on the upper leaf surface lure tiny insects. The prey are mired down in sticky secretions. Then glandular hairs bend to deliver prey to the center of the leaf blade, where digestion occurs using enzymes from other types of hairs.
- Example: butterworts, Pinguicula. Prey are mired down in the sticky substance from glandular hairs. When captured, the digestive glands secrete enzymes. Some species have inrolling of leaf margins.
- Example: Byblis. Long, thin leaves are covered with sticky, glandular hairs.
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