The Sundew Plant
The Sundews (Drosera) comprise one of the largest genera of carnivorous plants, with over 170 species. These members of the family Droseraceae lure, capture, and digest insects using stalked mucilaginous glands covering their leaf surface. The insects are used to supplement the poor mineral nutrition that sundews are able to obtain from the soil they grow in. Various species, which vary greatly in size and form, can be found growing natively on every continent except Antarctica.
Sundews are perennial (or rarely annual) herbaceous plants, forming prostrate or upright rosettes between 1 centimeter (0.4 in.) and 1 meter (39 in.) in height, depending on the species. Climbing species form scrambling stems which can reach much longer lengths, up to 3 meters (10 ft.) in the case of D. erythrogyne. Sundews have been shown to be able to achieve a lifespan of 50 years. The genus is so specialized for nutrient uptake through its carnivory that in at least the case of pygmy sundews it is missing the enzymes (nitrate reductase in particular) that plants usually use for the uptake of earth-bound nitrates.
Sundews are characterised by the glandular tentacles, topped with sticky secretions, that cover their laminae. The trapping and digestion mechanism usually employs two types of glands: stalked glands that secrete sweet mucilage to attract and ensnare insects and enzymes to digest them, and sessile glands that absorb the resulting nutrient soup (the latter glands are missing in some species, such as D. erythrorhiza). Small prey, mainly consisting of insects, are attracted by the sweet secretions of the peduncular glands. Upon touching these, however, they become entrapped by sticky mucilage which prevents their progress or escape. Eventually, the prey either succumb to death through exhaustion or through asphyxiation as the mucilage envelops them and clogs their spiracles. Death usually occurs within one quarter of an hour. The plant meanwhile secretes esterase, peroxidase, phosphatase and protease enzymes. These enzymes both dissolve the insect and free the contained nutrients. The nutrient soup is then absorbed through the leaf surface and can then be used to help fuel plant growth.
All species of sundew are able to move their tentacles in response to contact with digestible prey. The tentacles are extremely sensitive and will bend toward the center of the leaf in order to bring the insect into contact with as many stalked glands as possible. According to Charles Darwin, the contact of the legs of a small gnat with a single tentacle is enough to induce this response. This response to touch is known as thigmotropism, and is quite rapid in some species. The outer tentacles (recently coined as "snap-tentacles") of D. burmannii and D. sessilifolia can bend inwards toward prey in a matter of seconds after contact, while D. glanduligera is known to bend these tentacles in toward prey in mere tenths of a second! In addition to tentacle movement, some species are able to bend their laminas to various degrees in order to maximize contact with the prey. Of these, D. capensis exhibits what is probably the most dramatic movement, curling its leaf completely around prey in 30 minutes. Some species, such as D. filiformis, are unable to bend their leaves in response to prey.
By: Zookeeper - 2007-05-19 00:00:18