Carnivorous Plants Essay, Research Paper
Carnivorous Plants
In a world where plants are at the bottom of the food-chain, some
individual plant species have evolved ways to reverse the order we expect to
find in nature. These insectivorous plants, as they are sometimes called, are
the predators , rather than the passive prey. Adaptions such as odiferous lures
and trapping mechanisms have made it possible for these photosynthesizers to
capture, chemically break-down and digest insect prey (and in some cases even
small animals.) There is no reason to fear them though. The majority are
herbaceous perennials, usually only 4 to 6 inches high, and nothing like the
plant in “Little Shop of Horrors”.
Almost all carnivorous plants have a basically similar ecology and
several different species are often found growing almost side by side. They are
most likely to be found in swamps, bogs, damp heaths and muddy or sandy shores.
Drosophyllum lusitanicum from Portugal and Morocco is the one exception, it
grows on dry gravelly hills. Like other green plants, carnivorous plants
contain the organic pigment chlorophyll. This pigment helps to mediate a
chemical process called photosynthesis. This converts light energy into the
chemical bond energy of carbohydrate which is utilized as cellular energy, plant
growth and development. Water, carbon dioxide, nutrients and minerals are also
needed for survival. In wetlands, where stagnate water contains acidic
compounds and chemicals from decaying organic matter many plants have a
difficult time obtaining necessary nutrients. It is in these nutrient poor
conditions that some plants evolved different ways of obtaining nutrients. The
ability of carnivorous plants to digest nitrogen -rich animal protein enables
these plants to survive in somewhat hostile environments.
The evolution of carnivorous plants is speculative due to the paucity of
the fossil record. It is believed that plant carnivory may have evolved millions
of years ago from plants whose leaves formed depressions that retained rain
water. Small insects would sometimes fall into these water reservoirs and drown,
eventually being decomposed by bacteria in the water. The nutrients from the
insects would be absorbed by the leaf. The deeper the leaf depression the more
insects that could be drowned. This would have created a distinct survival
advantage allowing some plants to better compete in nutrient poor soil. As time
passed, these plants would evolve more effective trapping mechanisms.
There are more than 500 known species of carnivorous plant, although
some are now extinct. Classification is done using the standard binomal system
and is based primarily on the floral characteristics of the plants, not the
trapping mechanisms. They are divided into two groups based on corolla
structure; Choripetalae and Sympetalae. The group of plants categorized as
carnivorous belong to seven families, which are recognized by the suffix ?aceae’,
and fifteen genera. More than half of the species belong to the family
Lentibulariacene that is marked by bilaterally symmetrical flowers with fused
petals. The remainder of the species belong to six families marked by radially
symmetrical flowers with separate petals. Classification is illustrated in the
chart below in addition to the geographic range, the number of species, and the
type of trapping mechanism.
Family Genus species Geographic
Distribution Type of Trap Byblidaceae Byblis
2 Australia Passive flypaper
Cephalotaceae Cephalotus 1 S.W.
Australia Passive pitfall Dioncophyllaceae
Triphyophyllum1 West Africa Passive flypaper
Droseraceae Aldrovanda 1 Europe, Asia,
Africa, Australia Active
Dionaea1 North & South Carolina
Active steel Family Genus # of species
Geographic Distribution Type of Trap
Drosera 120 Omnipresent Passive
flypaper
Drosophyllum1 Morocco, Portugal, Spain
Passive flypaper NepenthaceaeNepenthes 71
East Indies Passive pitfall Sarraceniaceae
Darlingtonia1 California & Oregon, Passive
pitfall
Western Canada
Heliamphora6 North and South
America Passive pitfall
Sarracenia9 North America
Passive pitfall LentibulariaceaeGenlisea 14
Tropical Africa and Passive lobster
South America,
Madagascar
Pinguicula 50 Northern
Hemisphere and Passive pitfall
South America
Polypompholyx2 Australia
Active mousetrap
Ultricularia 300 Omnipresent
Active mousetrap
In the above chart, it can be seen that there is a large number of
different types of traps. The modified leaf traps
can be categorized as either active or passive. An active trap is one that
employs rapid movement as an integral part of the trapping mechanism, a passive
trap does not use rapid movement.
Active traps are categorized as “steel” or “mousetrap”. Active steel
type traps consists usually of two rectangular lobes that are hinged on one side.
The two lobes move rapidly toward each other to entrap prey when stimulated.
Active mousetraps are suction traps that use egg-shaped leaves or bladders that
have an opening with a door on one side. When trigger hairs on the door are
touched the leaf releases pressure and sucks the prey into the trap. In the
aquatic species of the genus Utricularia, this is the most complex and rapidly
acting trap; prey is sucked up into the bladders in 1/30 of a second.
There are three types of passive traps; “pitfall traps”, “lobster traps”,
and “flypaper traps”. Not completely passive, Lobster traps employ slow moving
tentacles that are powered by cell growth. These plants lead prey into their
trap using these two hairy spiral arms to guide the prey. Many plants capture
prey by forming clever containers creatures enter but can not escape from.
Passive pitfall traps, such as the ones employed by the butterworts (genus
Pinguicula) and pitcher plants (Darlingtonia, Sarraceniaceae, & Nepenthes),
attempt to lure insects into their cylindrically shaped hollow vessel and into
it’s stomach, which is often referred to as the pitcher. The insects get stuck
in the digestive enzymes of the pitcher and die. Flypaper traps, such as the
sundew (Drosophyllum & Drosera), produce sticky mucilage that covers the upper
surface of its leaves. Insects become mired in this and leaves then bend around
or roll up to enclose the prey for digestion.
Within the carnivorous plant world there are some truly amazing plants.
Of all the hundreds of species Dionaea muscipula, the Venus fly trap, is
probably the most dramatic. It is the only species in it’s genus and there are
no other plants quite like it. It’s hinged leaf lobes are capable of snapping
shut on prey in less than a half a second, eventually crushing the insect. Like
many carnivorous plants, the Venus fly trap lures prey in with bate, which in
this case is the smell of nectar. When an insect enters one of the bizarre
traps it might bend one of the three stiff trigger hairs in the center of the
leaf. When bent a couple of times in succession these hairs activate the trap.
The plant does not have muscle tissue, the process of closing instead involves
electrical signals and changes in water pressure. The book The Nature of Life
briefly describes the process of the Venus fly trap closing once triggered by
saying that:
trigger cells at the foot of the hair are deformed, as if pried
by a lever. Stimulated by the stress, trigger cells generate an
electric signal that flows from cell to cell through the leaf.
Specialized motor cells receive the signal, change shape, and cause
the trap to close.
About ten days are needed for digestion after which the leaf slowly opens up
again revealing only the undigestible chitin remains. The trap, not the plant
itself, turns black and dies when the plant tries to digest fats or eventually
after three or four captures.
The largest carnivorous plants belong to the genus Nepenthes. The vines
of these plants are usually tens of meters long. This genus is also capable of
catching some of the largest prey in their pitchers, including creatures as
large as frogs and small rodents. Nepenthes are unique amongst carnivorous
plants as the only dioecious genus, which means there are separate male and
female plants. These plants are very endangered and several species or extinct.
Some species of Nepenthes are sold for hundreds of dollars to collectors and are
involved in illegal overseas trade.
The growing of carnivorous plants has become very popular in recent
years. Unfortunately the endangered status of many species does not stop
collectors from risking high fines and field collecting them. This has had
seriously impact on many species, but collectors are not the biggest problem
facing carnivorous plants. In the USA and other developed countries wetlands
are considered useless and are being drained and developed on. At present it is
estimated that only 3-5% of carnivorous plant habitat remain in the US. Another
problem is that fires are put out before they spread even though many plants,
such as the Venus fly trap, benefit from periodic burns. Habitat destruction
from slash and burn agriculture, however, does not benefit any of the
carnivorous plants and is also causing a great deal of the extinctions.