The Venus Flytrap Dionaea muscipula

BOTANICAL NAME: Dionaea muscipula Ellis ex L. A monotypic genus; that is, there is only one species in the genus. Family Droseraceae.

COMMON NAMES: Venus' flytrap, flycatcher, tip-itiwitchet, catch-fly sensitive. (The latter two names are ancient.)

RANGE: Quite localized in scattered savannahs of southeastern North Carolina and neighboring eastern South Carolina in an approximate landward radius of 60-75 miles around Wilmington, N.C.

FLOWERING SEASON: Late May through early June.

TRAP SEASON: Some plants remain evergreen in protected situations, while many die back for the winter. New traps begin growing in March and continue into October.

DESCRIPTION. — The plant is a rosette of leaves that radiate out from a central point, the leaves being totally or partially reclining. The rosette measures 10-14 cm across when mature. The leaves arise from a somewhat elongate, fleshy, white rhizome (underground stem) often miscalled a "bulb." The rhizome elongates and

Fig. 2-1. A plant of Dionaea muscipula as typically seen in the field.

enlarges annually. Fibrous roots descend 10-15 cm. The green leaves grow up to 12 cm long. They are of two parts: a narrow to relatively broad leaf like petiole (leaf stem) nearer the rosette center, and a leaf blade modified into a unique trap, measuring up to 3 cm long.

- The flowers are on a 1-30 cm scape. They are acti-nomorphic, have white petals, and are about 1.0-1.5 cm across. After fertilization, tiny, black, pear-shaped seeds set (mature) in 6-8 weeks. These will germinate immediately when sown on a suitable substrate; storage at warm temperatures results in a lower rate of germination. The plants mature from seedlings to flowering age in 3-4 years, and estimates of the age of the oldest known living plants are nearly 25 years.

The traps consist of two clamshell-like halves. Around the free margins (unattached edges) are numerous stout guard hairs and minute nectar glands. The trap is normally in a 45°-60° open position when undisturbed. The interior of each half is lined by nearly microscopic digestive glands, which give the surface a finely granular or cobblestoned appearance. Each inner half has also three smaller, finer trigger hairs in a triangular pattern (rarely anomalous plants have up to six hairs per half) which, when properly stimulated, initiate trap closure. The trap lining is colored variably green to pale yellow to bright red with frequent intermediate shades and patterns noted.

The usually bright coloration and the secretions of sweet nectar by the marginal glands may attract prey to the interior of the trap, where the insect brushes against one or more of the trigger hairs. Initially, trap closure is quite rapid until the guard hairs mesh, effectively incarcerating the small prey in a barred sarcophagus. The slower, secondary phase of closure results in the margins sealing tightly together so that the whole trap becomes a flattened, stomachlike pouch. At this stage, the margins of the trap halves evert slightly. If live prey—not a raindrop or a piece of windblown debris —has been caught, digestive fluids are then secreted into the interior of the closed trap. Apparently, the struggles of the prey and certain

Fig. 2-2. A plant of more upright habit. Note the numerous red-lined traps with prominent marginal hairs.

Fig. 2-3. Close view of two traps, one with previously digested insect remains, the other after the rapid phase of closure. The intef/neshing guard hairs hold the prey in until the slower closing phase is completed.

chemical compounds that emanate from the prey (such as amino acids) stimulate more copious secretions of digestive fluid with greater concentrations of enzymes. Digestion then occurs, and the nutrients are absorbed at the bases of the glands over a period of 3-5 days, depending on the temperature, the size of the prey, its nutritive value, etc. Afterwards, the trap reopens. The dry, chitinous remains of the insect stay in the trap or drop out.

Each trap may be stimulated mechanically by touch to close about ten times before it will no longer respond. After such closures, the trap reopens the next day, since there is no animate matter to digest. If the prey is not too large, each trap may catch and digest up to three times, after which it ceases to function. Very large catches result in the death of the trap leaf, but new ones grow more or less continuously all season.

In order for the trap to close, any one of the trigger hairs on the inner surface must usually be touched twice, or any two hairs must be touched once each in succession. Wt temperatures above 40°C, however, one stimulus suffices in half the cases. The longer the period between the two stimuli, the slower the closure. The very quick reaction no longer occurs if the period exceeds the range of 20-40 seconds. If the stimuli are farther apart, closure will eventually occur, but it is extremely slow and multiple stimuli are required during closure to complete the process. I It is felt that the struggle of the live prey inside the trap continuously stimulates the closure mechanism and the secretion of

Fig. 2-4. Interior of trap. Note the three triangularly spaced trigger hairs on each inner surface of the trap.

Fig. 2-5. Appearance of trap after catching a live insect, the slow phase of closure having been completed. The edges are tightly sealed.

the digestive enzymes until the very tight, pouchlike seal is formed.

The mechanism of closure is not entirely understood. A small but consistently measurable action potential, or electric current, crosses the leaf after the trigger hair is stimulated. Sometimes, the stimulation of parts of the leaf other than the trigger hairs initiates the action potential and results in closure. The fact that the total number of closures for any one trap is limited indicates that some cell growth function, metabolic process, or both, is ultimately exhausted. The fact that the number of repeat closures is more limited if prey is caught- each time certainly indicates that the process is accumulative rather than exhaustive; that is, that possibly the storage or metabolic manufacture of some product — perhaps starch or protein —may cause the process of closure to become inhibited sooner than it would with inanimate stimulation.

In spite of initial appearances, the trap does not close like a bear trap. The two halves do not rotate on the midrib like a hinge. When open, the outer surface of each trap half is concave, or dished in, while the inner surfaces are bulging inwards. During closure, these surface conformations are reversed, so that the free edges are quite suddenly brought closer together — close enough for the strong guard hairs to intermesh.

Fig. 2-6. A cluster of plants in flower. The scapes are disproportionately long for the size of the plant, perhaps so that the flowers are raised above the grasstops for effective pollination, and so that the potential pollinators are not more tempted by the traps farther down.

Fig. 2-7. Flowers of Dionaea muscipula.

Fig. 2-8. Typical view of Dionaea flowering in the field. The small white flowers are just visible above the grasstops, while the vegetative parts are partially obscured.

This process can be noted by close, careful observation as well as by high-speed comparison photographs or motion pictures. However, the change in surface conformation explains only the rapid phase of closure. During the slower, sealing phase, no further significant change in surface conformation is observable except for the eversion of the very margins, and there is even some slight loss of the new convexity of the outside surfaces. At this stage, some hingelike rotation on the midrib likely occurs.

As we mentioned, the bladelike leaf petiole has a variable morphology. In shadier locations, or in the early spring prior to flowering, the petiole is quite thin and wide and later ones become thicker and narrower. But even late in the season and in full sun, some petioles of adjacent plants remain wide, while others become almost stemlike or triangular in cross section. Both variations can sometimes be found in plants in a single location. Some observers feel that these variations are inherent in different genetic strains of the species, and they have recognized at least four such variations, including the two extremes and two intermediates. Another interesting observation is that the narrower the petiole, the more erect the whole leaf tends to be.

The color of the trap lining may follow a similar pattern. Generally, growth in bright sunlight brings out the brightest red color. However, some plants growing right beside the red ones, in the same light and soil, remain green or yellowish or are even variegated red and green.

Anomalies of morphology aside from that of the petiole are also occasionally noted. We have mentioned that the trap can have up to six trigger hairs per half.

Fig. 2-9. Seed, which ripen exposed. Fig. 2-10. Seedlings.

Another individual, nongenetic variation is a double trap on one petiole. An occasional flower anomaly is vegetative apomixis, in which the flower parts— sepals, petals, stamens, and pistil—are replaced by miniature plantlets which can be rooted and grown to normal plants. This occurs mainly when spring weather has been uneven during the early period of flower bud initiation, with cold nights alternating with warm, sunny days. The author produced this phenomenon some years ago in Ohio by growing the plants on a window-sill during March. The warm sunlight streaming through the window initiated early growth and flower budding, but the nights were so cold that the glass next to the sill was often frosted on the inside in the mornings. It is likely that other such environmental shocks, perhaps involving chemical substances such as one or more of the plant hormones, could precipitate the process. Finally, even the widened, bladelike petioles are capable of producing vegetative buds. The process has been utilized in culture in order to propagate the plants rapidly, and the phenomenon has occasionally been observed in nature.

GENERAL.— Largely because of habitat changes, Dionaea muscipula is markedly decreasing in numbers throughout a shrinking range which is none too large to begin with. Indeed, earlier reports document a far more extensive range in the Carolinas than we are able to report now. Dionaea does tend to remain on a deteriorating site longer than many associate carnivorous and noncarnivorous plants, particularly pitcher plants (Sarracenia). The plants grow in moist, sandy, acid savannahs among wire grasses, sedges, and many native orchids (Pogonia, Calopogon, Platanthera, Spi-ranthes, etc.) between rather widely spaced longleaf pine trees. Dionaea will tolerate short periods of drought and flooding, submerged plants having been observed catching small aquatic animals. Since the underground stem of the Venus' flytrap is well protected, Dionaea is among the first plants to sprout back strongly in a burned area. If the area is not burned from time to time (or if the water table drops), other herbs, shrubs, and trees encroach and quite quickly crowd out the smaller Dionaea, since the habitat is then a completely different one. Thus a rapid surface fire in the autumn is actually quite beneficial.

In late summer, because the neighboring grasses are quite tall by that time, considerable search is required by the uninitiated before he finds the plants, often after walking over them for some time! The best time to observe Dionaea is in early spring when grasses and sedges are shorter, and especially when the flowers, lifted by the tall scapes above the grass tops, can be seen easily even from an automobile.

Dionaea is rather hardy climatically. Outdoor experimental transplants have thrived as far north as New Jersey and some bogs in Pennsylvania. Incidentally, such transplants, which were often conducted without fanfare to discourage vandals, have nearly led to some embarrassing results when skilled naturalists, unaware of the experiments, have come upon the plantings during walks and nearly rushed to publication with the news of Dionaea's supposed range extension.

While there is certainly much more to learn about all the plants in this book, Dionaea muscipula undoubtedly remains the pet among most students of carnivorous plants, botanists in general, and naturalists of all walks.

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