Probably the most famous of all carnivorous plants
because of its swift movement of trap leaves, the Venus flytrap is endemic to
the Atlantic coastal plain of North America, where it is highly localized to
southeastern North Carolina and the adjacent
northeastern South Carolina, within a 100km radius
around the city of Wilmington on the N.C. coast.
The plants are typically found on a moist surface in the
open pine tree forest, sometimes on a mat of sphagnum moss, sometimes
right on a white sand surface, often in company of other carnivorous
plants in the area such as sundews and terrestrial bladderworts.
The genus name Dionaea is from Dione, in
Greek mythology. Venus, in Roman religion, is identified with the Greek goddess,
Aphrodite, whose mother, according to one account, is Dione.
The genus Dionaea
is monotypic; that is, there is only one species, Dionaea
muscipula. The genus belongs to the sundew family Droseraceae.
In a historical letter dated Jan. 24, 1760, Arthur Dobbs,
of North Carolina -- who appears to be the first discoverer of the
plant, at least as far as historical records go -- wrote, " ...But the
great wonder of the vegetable kingdom is a very curious unknown species of
sensitive; it is a dwarf plant; the leaves are like a narrow segment of a
sphere, consisting of two parts, like the cap of a spring purse, the concave
part outward, each of which falls back with indented edges (like an iron spring
fox trap); upon anything touching the leaves, or falling between them, they
instantly close like a spring trap, and confine any insect or anything that
falls between them; it bears a white flower; to this surprising plant I have
given the name of Fly Trap Sensitive."
In 1770, based on living plants in Kew brought back by the
Queen's botanist, a London merchant, Ellis, was the first to publish a
carnivorous habit of the plant. Ellis' enthusiasm is expressed in the letter
sent to Linnaeus, " ... the plant, of which I now inclose you an exact
figure… that nature may have some view toward nourishment, in forming the
upper joint of the leaf like a machine to catch food: upon the middle of this
lies the bait for the unhappy insect that becomes its prey. Many minute red
glands, that cover its inner surface, and which perhaps discharge sweet liquor,
tempt the poor animal to taste them; and the instant these tender parts are
irritated by its feet, the two lobes rise up, grasp it fast, lock the rows of
spines together, and squeeze it to death. And further, lest the strong efforts
for life, in the creature thus taken, should serve to disengage it, three small
erect spines are fixed near the middle of each lobe, among the glands, that
effectually put an end to all its struggles…" (Lloyd, 1942).
Linnaeus allegedly did not share Ellis' enthusiasm and
regarded the movement as merely a similar irritability as in Mimosa, and
believed that the captured insect was released upon reopening of the lobes.
The plant is a perennial rosette of leaves emerging from a
short white rhizome (underground stem) with fibrous roots. The leaves, which
grow up to 10cm or so, are of two parts: a flat petiole (leaf stem) and a leaf
blade modified into a trap. The trap portion consists of two semicircular lobes
united along the midrib of the leaf blade. Around the margin of each lobe grow
stiff, spine-like guard hairs (15-20 along each margin), which mesh together
when the trap closes to prevent prey's escape.
Along the inner edge of each lobe just below these marginal spines runs a narrow
band of nectar glands. Much of the inner lobe below is crowded with numerous
digestive glands, which often give the trap surface a bright-red coloration.
Each inner lobe normally has three --
sometimes 4 or more -- fine
bristles located in a triangular pattern. These are trigger hairs, which are
sensitive to physical stimulation and, when properly stimulated, initiate a
rapid trap closure.
Multiple Stimulation Requirement
Many studies have been conducted on the snap trap
mechanism of this plant. In a normal condition, stimulating a trigger hair once
does not cause a trap movement. To initiate a successful closure, it is
necessary to stimulate two different hairs, or stimulate the same hair twice,
within a 20 seconds or so interval. If the two stimuli are given in a rapid
succession, a quite swift closure -- well under half a
second -- immediately follows. The longer the interval
between two stimuli, the less rapid the closure becomes. If it is within 20
seconds, a rapid trap closure usually occurs within a few seconds. If the second
stimulus takes place after a half minute or later, however, the closure itself
becomes extremely slow and multiple stimuli may be required to complete the
closure. On the other hand, if the temperature is 35 degrees C or higher, a trap
closure may occur by one stimulus only. It is also noted that the stimulation of
the lobe part, other than the hairs themselves, particularly the area near the
hairs, does sometimes induce trap closure. It is also observed that a mild rain
usually does not trigger the trap.
This build-up of a threshold potential before a trap
motion is actually triggered is considered advantageous for the Venus' flytrap.
This ensures a successful catch in many cases, by
properly positioning the prey on the center of the trap plate before a trapping
attempt, rather than closing the trap prematurely when much of the insect's
body may still be outside the trap.
Studies have shown that sensory cells are located near the
base of the trigger hair, at the indentation where bending strains are most
pronounced upon stimulation. Action potentials have been observed upon bending
of the trigger hairs.
Digestion and Absorption
The initial phase of rapid closure is followed by what is
termed the narrowing phase. This second phase is pursued only if the trap
catches something nutritious to the plant, or if the stimulus is continuously
coming as would be the case if a live insect is captured. The narrowing phase is
a slower process which brings the two lobes tightly together. Often the pressure
exerted in this phase is strong enough to crush a soft-bodied insect prey, thus
possibly inducing chemical stimulation.
As the lobes are tightly sealed, digestive fluids are
secreted from the digestive glands into the closed trap interior. The prey, if
still alive, will most probably be suffocated in the fluids. The digestive
secretions which is known
to be initiated only by proper chemical stimulation typically begins a day or so after the capture of prey,
although such protein-rich substances as egg-whites may often induce copious
secretions in a matter of a few hours. Digested material is then absorbed through
the glands and through the inner surface of the trap.
The digestive process, not unlike that of an animal's
stomach, lasts for a few days to a week, depending on the size of the prey. When
the digestion/absorption process is over, the trap
(now dry) reopens, revealing the chitinous remains of the
insect prey. The wind and rain clean the lobes, and the trap is ready again for
another meal. Each trap is capable of catching a prey two or three times at most
before it ceases to function. Too large a catch often results in the damage of
the trap, as the trapped animal begins to decay,
causing the entire leaf blade to blacken and die out before the digestion is
complete -- an upset stomach, as it were.
Both the nectar and the digestive glands are identical in
structure, though distinct in function, with each gland consisting of about 32 glandular cells. The digestive
glands are a little larger and are far more numerous than the nectar glands.
Both are sessile glands (stalkless). Nectar
glands are colorless and almost buried on the trap surface to allow tight
sealing during the narrowing phase. Typically, the digestive glands are
pigmented which color much of the inner surface with a bright red hue.
In the early spring, a tiny bud appears in the rosette
center, which develops into a tall scape bearing elegant white flowers. An
actinomorphic flower with five petals bloom in May through June in North
Carolina. A scape often reaches a height of 20-30cm in nature, providing a
possible separation of pollinators and prey. Numerous black seeds
fully mature by late July. As the seed capsule dries, the pressure from capsule
shrinkage bursts the slippery, pear-shaped seeds around the field. In nature,
the seeds germinate shortly after the dispersal and tiny seedlings have a few
months to prepare for the onset of winter. It takes 3 to 4 years for the
seedling of the Venus'
to reach a flowering age.
Cost of Carnivory
updated: January 15, 2006