sarracenia special adaptative features and where they generally found
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Insectivorous Plants
Lewis J. Feldman, in Encyclopedia of Insects (Second Edition), 2009
Passive Traps
Perhaps the most familiar examples of passive traps are the sundews (Drosera) and the pitcher plants (Sarracenia and Darlingtonia in temperate climes and Nepenthes in the tropics). The sundews capture their prey by producing from stalked glands an adhesive, or glue (the drop of “dew”), which captures and holds fast the insect. As the prey struggles, it is covered with the sticky mucilage and, as a consequence, suffocates. The stalked glands then bend in toward the prey; in some species, the entire leaf enfolds the prey. A second type of gland on the leaf secretes digestive enzymes and acids, initiating the breakdown and subsequent absorption of nutrients. Darwin was so enthralled with the sundews that about two-thirds of his book Insectivorous Plants is devoted to this group. He notes his surprise at “finding how large a number of insects were caught by the leaves of the common sundew,” and speculates that, “as this plant is extremely common in some districts, the number of insects thus annually slaughtered must be prodigious.”
A fascinating variation on the carnivorous plant passive trap theme is shown by plants that comprise the genus Roridula. These plants, considered by some workers not to be truly carnivorous, are native to South Africa and may be near extinction. Individuals in this group have leaves covered with stalked mucilage-secreting glands, which as in the sundews capture and hold fast insects. This is where the carnivorous story would end, were it not for another player, a species of assassin bug, which forms a mutualistic association with Roridula. Large numbers of these predaceous bugs may inhabit Roridula and are able to traverse the leaves, without themselves being ensnared by the glue. When other insects are captured by the plant, the assassin bugs move to the trapped prey, suck out their liquid contents, and, some time afterward, defecate a nutritious substance that is absorbed by the leaf and nourishes the plant.
The second major group of plants that have passive traps are the pitcher plants. In this group, the leaf becomes variously modified, often into a tube, and develops at the base of the tube a well that must fill with water for the pitcher to function as a trap. The temperate species of pitcher plants (Darlingtonia in the western United States and Sarracenia in eastern North America) (Figs. 1 and 2) are usually terrestrial. In these plants, the leaf lures flying insects by producing nectar, in some plants laced with volatile insect attractants (e.g., enol diacetal monoterpene and/or a series of methyl esters) that sometimes covers the colorful appendages. Crawling insects follow nectar trails running along the outside of the leaf. The nectar trails lead to the mouth of the tube, where the surface is smooth and slippery and from which the insect can easily lose its foothold, thus falling into the watery well. Escape from the well is almost impossible, since the inside wall of the tube leaf is lined with downward-pointing hairs. One would think that flying insects could fly out if they started to fall. To counter this possibility, pitcher plants such as Darlingtonia have developed a hooded leaf, transparent and sealed at its top. When an insect tries to leave the leaf, it flies toward light coming through the transparent upper portion of the hood. Since, however, the exit is sealed, eventually the insect becomes so exhausted that it falls into the well. There is some suggestion that pitcher plants may produce a “drug” to confuse the flying insect, and that fluids in the well may contain substances (in Sarracenia flava, a toxic alkaloid, coniine, [the poison in hemlock]) that is suggested to quiet and stun the fallen insect. In addition to nectar serving as an attractant, the possible development of ultraviolet signaling, as employed by flowers to attract pollinators, may also serve to lure insects to the trap. Many temperate pitcher plants secrete hydrolytic enzymes into the liquid in the well, thereby digesting the insect, whereas other pitcher plants (e.g., Darlingtonia) produce none of their own digestive enzymes but instead rely on bacteria to decay the insect. In either model, the digestive enzymes can be quite powerful, with only the hardest parts of insects, such as legs or shells, remaining undigested.
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