The Types Of Traps
Contrary to some portrayals in science fiction, the flowers of carnivorous plants are not the organs that ensnare their prey. Rather, in all cases the deadly traps are modified leaves and stems. There are three basic types of trapping organs: active, adhesive, and passive.
Active traps of carnivorous plants attract their mostly arthropod prey using various machinations, including color, scent, and nectar. Once a victim is suitably within, the trap rapidly closes, preventing the escape of the prey. The active trap of the Venus fly-trap (Dionaea muscipula) is modeled on a basic clamshell design. This species utilizes a fast-acting response to a mechanical stimulus caused when an insect triggers sensitive hairs in the trap, causing its clam-shell leaves to close. The fringing outer projectiles of the leaves rapidly enclose to form a barrier that prevents the trapped arthropod from escaping. At the same time, mechanical stimuli from the struggling victim trigger the synthesis and excretion of digestive enzymes onto the inner surface of the trap, which facilitate digestion of the prey.
Another design of active trap is based on a small, hollow chamber with a trap door. This design is utilized by the bladderworts (various species of Utricularia), small aquatic plants that form little bladders with diameters of several millimeters, that trap tiny aquatic invertebrates behind a rapidly closing trap door. The door of the bladderwort trap initially swings quickly into the bladder, triggered to respond in this way by motion sensed by fine, fringing bristles. The inward motion of the door develops a suction that can sweep invertebrates into the trap, where they are trapped by the re-closing door, and are digested for the nutrients they contain.
Adhesive, semi-active traps primarily rely on sticky, surface exudates to ensnare their prey. Once a victim is firmly entangled, the leaf slowly enfolds to seal the fate of the unlucky arthropod, and to facilitate the process of digestion. This manner of trap is typified by the most species-rich of the carnivorous plants, the genus of plants known as sundews (Drosera spp.). These plants develop relatively wide, modified leaves, that are densely covered with stalked glands that resemble tentacles several millimeters long. Each tentacle is tipped with a droplet of sticky mucilage. Unwary arthropods, lured by scent, color, and nectar, are caught by this gluey material and are then firmly entangled during their struggles. The leaf then slowly, almost imperceptibly, enfolds the prey, which is then digested by proteolytic enzymes secreted by special glands on the leaf surface.
Passive traps lie in deadly wait for their small victims, which are attracted by enticing scents, colors, and nectar. However, these seeming treats are located at the end of a fatal, usually one-way passage, from which the prey cannot easily exit. The passage terminates in a pit filled with water and digestive enzymes, where the victim drowns, or is attacked by predacious insects that live symbiotically with the carnivorous plant.
The ingenious design of the pitcher plant (Sarracenia purpurea) is a revealing example of passive traps. The pitcher plant has foliage modified into upright vessels, as much as 4-6 in (10-15 cm) tall. When mature, these are reddish-green in color, with ultraviolet nectar guides pointing into their interior, which also emits alluring scents. The fringing lip and upper part of the inside of the pitcher are rich in insect attracting nectaries (organs that secrete nectar), and are covered with stiff, downward pointing bristles. These bristles can be easily traversed by an insect walking into the trap, but they passively resist movement upwards and out of the trap. Beneath the zone of bristles is a very waxy, slippery zone, the surface of which is almost impossible for even the tiny feet of insects to grasp, so they fall to the bottom of the trap. There the victim encounters a pool of collected rainwater, replete with digestive enzymes and the floating
corpses of drowned insects, in various stages of decay and digestion. The newest victim struggles for a while, then drowns, and is digested.
Interestingly, a few species of insects are capable of living happily in the water-filled vessels of the pitcher plant and related species, such as the cobra plant (Darlingtonia californica). These insects are resistant to the digestive enzymes of the carnivorous plants, and they utilize the pitchers as a micro-aquatic habitat. Some species of midges and flies that live in pitcher plants actually attack recently trapped insects, killing and feeding on them. Eventually, the carnivorous plant benefits from nutrients excreted by the symbiotic insects. These pitchers also support a rich microbial community, which are useful in the decay of trapped arthropods, helping to make nutrients available for uptake by the carnivorous plant.
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