The plant consists of a short unbranched rhizome, which is a bulb-like structure formed by the overlapping basal portions of leaves surrounding the growing point. The root system is not extensive. The blade portion of each leaf is modified into a trap, by which the plant captures its prey, while the basal portion, the petiole which supports the trap, is fleshy and stores food. The total length of the leaf may be up to 8 in. (20 cm) long. The leaves are arranged in a rosette, forming a circular pattern around the growing point. New leaves originate from the growing point at the center of the rosette and are protected by the overlapping expanded basal portions of the older leaves. (Fig. 2-1)
Leaf characteristics vary with the season. Spring leaves tend to be green with broad petioles, whose lateral extensions are referred to as wings. The spring leaves reach lengths of 23A in. (7 cm) with a width of 3A in. (2 cm) at their broadest point. They are either erect or prostrate. Red coloration is absent or relatively limited, but when present it is usually restricted to the glands on the inner surface of the traps. Production of spring leaves is terminated by flowering during late spring or early summer. After
flowering is complete, summer leaves are produced which are easily differentiated from spring leaves, as they can be as long as or longer than the spring leaves but are very narrow and almost wingless. The traps produced by a plant usually are largest on the summer leaves and smallest on the winter leaves. Summer leaves tend to grow vertically. Plants growing in intense light, either natural or artifical, develop traps whose inner surfaces, and at times even the marginal spines, are a solid deep maroon-red color. (Photo 2-1) Light intensity controls red coloration in a majority of the Venus Fly Trap plants, although in some it is genetically controlled. When these plants are growing side by side, exposed to the same high-intensity light, most will develop a deep red coloration, but a few may not. With the beginning of fall, winter-type leaves with smaller traps are produced. These leaves tend to be prostrate and the width of the petioles tends to be between that of the spring and summer leaves.
In protected areas of their natural habitat the plants tend to be evergreen, but in other areas frost can kill the leaves during the winter.
In the spring, May and June, the plant produces a tall scape (flower stalk), with white flowers. The scape bears from 1-15 white flowers. Each flower consists of 5 green sepals, 5 white petals, usually 15 stamens and 1 compound pistil. (Fig. 2-2) Terminal flower buds open first, followed by the sequential opening of the others further down toward the base of the scape with up to 4 flowers open at one time. Plants grown from bulbs that were kept in cold storage usually will bloom within 2-3 months of planting, regardless of the season. If seed is not desired, remove the scape as soon as it is visible so that energy consumed by the flowering process can be diverted to vegetative plant growth and development.
Prey is captured by the rectangularly-shaped trap which consists of two lobes united along the midrib of the leaf blade. The trap has been compared to a hinge, but this analogy is inaccurate because a hinge has an articulated joint, and the trap has no such joint. Under normal conditions the angle formed by the open trap is in the neighborhood of 40 to 50 degrees. The margins of the trap are studded with bristles which are sometimes referred to as cilia. On each of the two inner lobes of the trap are three trigger hairs arranged in a triangular pattern with the apex of the triangle directed toward the midrib. (Photo 2-2) In rare cases there are four trigger hairs on each surface and in some cases the hairs bifurcate. On the outer, abaxial surfaces of the trap lobes there is an abundance of star-shaped structures known as stellate trichomes. Covering most of the inner, adaxial, surface of the trap lobes are two kinds of glands: the alluring and digestive-absorptive glands. These glands are structurally identical. The digestive-absorptive glands are conspicuous because of their red coloration due to the water-soluble pigment, anthocyanin, which is present in the cell fluid. The alluring glands, which produce a sugary substance that has a pleasing odor to insects, are arranged along the outer margins of the trap. It has been proposed that this arrangement is by design to prevent insects which are too small from effecting closure, thus enabling the plant to conserve energy for trapping larger meals. According to this theory an insect which is less than xk in. (0.6 cm) long is too short to set off the trap as it dines on the nectar produced by the alluring glands.
In nature, trap closure is effected by insects touching the trigger hairs. Observations indicate that a single hair must be stimulated twice or two hairs stimulated in succession for closure to occur. Normally the interval of time between the two stimulations cannot be too short or too long. This time interval has been found to be from about 1 second to about 20 seconds; if the interval is longer, then, additional stimulations are necessary to initiate closure. There are other factors that influence the number of
Fig. 2-2 Dionaea flower has 5 sepals, 5 white petals, numerous stamens and one compound pistil.
stimulations necessary to effect closure such as: age of plant, ambient temperature, length of time since last closure and general health of plant. The higher the temperature the more likely that a single stimulation is sufficient. Some research indicates that at 59°F (15°C) two stimulations are needed, at 95°F (35°C) often one stimulation is sufficient, and at 104°F (40°C) only one stimulation is necessary in half of the cases. As temperature decreases the action becomes sluggish and eventually ceases. The temperature at which trap action ceases is quite variable.
Closure of traps can also be effected by electrical stimulation, hot water 149°F (65°C), chemicals such as ether and chloroform, and by rubbing, pinching or cutting the area above the midrib on either the adaxial or abaxial surfaces of the trap. Virtually the entire surface of the trap is sensitive to stimulation of one sort or another. The closure produced by chemical agents is slower and has a prolonged effect, resulting in the trap being less responsive to later physical stimulation. The trigger hairs are by far the most sensitive or receptive to stimulation. Studies indicate, when electricity is used to set off the traps, that as the strength of the electrical impulse increases, the number of impulses required decreases.
It has been known since the late 1800s, that when the trap is stimulated an electrical voltage is produced. This electricity is produced regardless of whether the trap closes or not. Recent studies involving the electrical phenomenon of Venus Fly Traps have been conducted by Stuhlman and Daren. One of their conclusions is: "The action potential (voltage) runs a course characteristic of mammalian nerves in normal physiological condition."
Under ideal conditions the trap closes with an alarming speed, a second or less. The rapidity of motion can be startling. Upon suitable stimulation a healthy trap will close. This initial closing is known as the closing or shutting phase. (Fig.2-3A) The marginal spines become loosely interlocked, turning the trap into a jail cell with spines for bars. Small insects are able to escape through the open spaces between the interlaced marginal spines. If the trap has been mechanically stimulated to close or if the prey is small enough to escape, the trap will reopen in about 24 hours without further waste of time and energy. If, on the other hand, suitable prey has been placed in the trap or small animals such as insects, mollusks (snails and slugs) and arachnids (spiders and daddy long legs) have sprung the trap, the trap progresses to the narrowing phase which commences about half an hour after stimulation. (Fig.2-3B) By comparing the diagrams of the two traps, (Fig.2-3A and 2-3B), the differences between the phases become evident. The change that occurs in the positioning of the trap and marginal spines is analogous to the difference between interlacing the fingers of your right and left hand in a relaxed position with the thumbs overlapping and the bases of the palms touching as compared to both palms being tightly pressed together with the fingers out straight. During the narrowing phase considerable energy is expended. Often the lobes become so tightly pressed together that the outline of the captured prey is visible, and soft-bodied insects are crushed. (Photo 2-3) Following trap closure, fluids containing digestive enzymes are secreted. The tissue just below the marginal spines forms a secure seal so that prey along with the digestive enzymes that are secreted by the trap intermingle within the confines of the trap interior. It is probable that the digestive fluids drown the prey, putting an end to their struggle if they were not previously crushed. After digestion and absorption have taken place, the trap opens, exposing the chitinous remains from insects and small crustaceans and other undigestable materials.
Traps will remain closed for 1-2 weeks, if suitable prey has been captured. Otherwise they usually open within a day. Reopening of the trap has been attributed to a differential rate of growth of the trap surfaces. The adaxial surface of the trap grows more than the abaxial surface resulting in the opening of the trap.
Even though studies do not indicate that the Venus Fly Trap must have prey to survive, some show that plants which are fed are healthier and produce more seed. A
Fig. 2-3 Initial closure is evidenced by interlacing of spines. During final single trap can usually capture prey 3—4 times before the trap ceases to function. An artifically stimulated trap can close and open many more times.
When the captured prey is of the correct size for the trap, digestion proceeds without decay. The formic acid present in the digestive fluid is believed to be a bactericide. If prey is too large or if fat is placed in the trap, the trap commences to decay and turns black instead of opening. The death of a trap does not mean the death of the plant, as new traps are constantly being formed at the center of the rosette during the growing season.
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