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THE ECOLOGY OF CARNIVOROUS PLANTS



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By Gregg Chamberlain SEPTEMBER 1988
Some plants like green thumbs all too well.
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Giant man-eating plants, like dragons, goblins, and other weird creatures, are staples of modern fantasy fiction and most traditional legends. In the realm of the AD&D® game, official ‘monster plants’ (such as the hangman tree, the choke creeper, and the giant sundew) are representative of the genre. Carnivorous plants are not truly mythical, though the real ones are not as monstrous as fantasy works portray them. The following article explains the ecology of fantasy carnivorous plants based on that of their smaller, factual cousins.
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In the real world, carnivorous plants are represented by the various species of sundews, pitcher plants, butterworts, bladderworts, rainbow plants, waterwheels, pink petticoats, and Venus’ flytraps. In the AD&D game, these carnivorous plants are represented by the giant sundew (Monster Manual II, page 116) and by the giant pitcher plant and the giant Venus’ flytrap (DRAGON® issue #89, .Creature Catalog,. pages C-14 and C-18).
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Most plants with which we are familiar sustain themselves through the process of photosynthesis. Chlorophyll, which is contained in the leaves of the plant and which gives green plants their color, transforms sunlight into chemical energy. This chemical reacts with water (which is absorbed through the roots) and carbon dioxide (which is taken in through the leaves) to produce carbohydrates. These carbohydrates, along with the minerals and trace elements absorbed through the root system, are then utilized by the plant for its growth and development. Mosses, lichens, mushrooms, ferns, and the like are considered lower members of the plant kingdom because they either do not photosynthesize or lack other characteristics of green plants (such as a proper root system, reproduction via seeds, fertilization through pollination, etc.).
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Carnivorous plants belong in the category of green plants, possessing all the requirements for classification as such plus two other traits which take them a step beyond most such plants: the ability to self-reproduce without pollination and (most importantly) the ability to actively prey on insects and other minute animals for food.
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Carnivorous plants are usually found in bogs, swamps, and freshwater marshes. The soil in these environments has a lower content of minerals and elements than is acceptable to most green plants. The lack of nitrogen, phosphorous, potassium, and calcium results from the high acid content of the water, which may be caused by frequent rains that leech minerals out of the soil. In warmer climates, this lack may be caused by a higher rate of bacterial decay which also uses up precious materials needed for plant growth. Plants such as the sundew, pitcher plant, and others have adapted to these poor growing conditions by evolving means for trapping and digesting living prey as supplements to their diets.1
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Despite the broad range of some species of carnivorous plants, they are on the whole limited by their specialization to their ecological niches - acidic bogs, marshes, and certain alkaline marls. In the northern temperate and subarctic regions, sphagnum bogs are a favored habitat. These bogs are the remains of ancient glacial lakes which have gradually become filled with decayed plant and animal remains. The stagnant waters are overgrown with moss and slowly become more acidic. The young bog then becomes the home of pitcher plants, sundews, and bladderworts, with butterworts growing along the sandy parts of the shoreline. A marl bog is formed by the seepage of spring water over a flat surface that has a foundation of limestone deposits. This results in the percolation of calcium carbonate throughout the water, making it more alkaline than normal and producing the same mineral-deficient conditions that exist in acidic bogs. Some pitcher plants and sundews have adapted to marl bogs. In more temperate climates, acidic bogs may develop beside old lakes and sluggish streams and springheads. The movement of water under these conditions is too feeble to prevent stagnation. Here may be found pitcher plants, sundews, and bladderworts. In still warmer areas, savannah or grass-edge bogs form in low, flat, or slightly sloping areas with sandy soil and a high water table. The predominant vegetation consists of grasses, sedges, and widely scattered long-needle pines. Under these conditions may be found pitcher plants, bladderworts, butterworts, sundews, and Venus’ flytraps.
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Besides their restricted habitats (an especially serious problem for the Venus. flytrap, which is confined to savannah bogs), carnivorous plants are threatened by the encroachment of more common green plants as the bog matures. By adapting to the mineral-poor conditions of the bog, carnivorous plants eventually change the bog by increasing the supply of nitrates, phosphates, and other minerals when those plants die and decay. As the acid level drops and the soil becomes richer and sweeter, other plants more accustomed to such growing conditions move in and crowd out the carnivorous plants.
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All flowering plants normally reproduce by cross-pollination of their flowers by insects or the wind.2 Carnivorous plants normally reproduce by this means, but they are also capable of reproducing themselves asexually.
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For instance during pollination, the seeds of the Venus’ flytrap are black and pear-shaped when it reproduces. The Venus’ flytrap’s asexual means of reproduction involves a fleshy, white, underground rhizome that elongates annually and from which new Venus’ flytraps may grow. This underground rhizome also makes it very difficult for fire and other natural disasters to completely destroy the plant. Where temperature conditions are subject to uneven fluctuations (alternating warm and cool spring days, for example), the plant can also reproduce itself by budding. Through this process, the flowers of the plant are replaced by miniature plants which take root around the ‘mother’ and grow normally. As a result, Venus’ flytraps may be found growing in colonies.
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The seeds of the pitcher plant are teardrop shaped and range in color from brown to pinkish gray. Like the Venus’ flytrap, this plant may reproduce asexually by means of a rhizome; resulting in colonies of pitcher plants connected to the mother plant). The giant pitcher plant in DRAGON issue #89 resembles the Australian pitcher plant, which can also reproduce asexually in a manner similar to that of the strawberry plant. The pitcher plant, in this case, possesses a root which acts as a central node for its thick, branching roots. Some of these roots form foliage leaves above ground in the shape of a rosette. In the fall, a pitcher plant embryo forms around the center of these rosettes. As the embryo grows, the runner leaf stalk lengthens, taking the embryo away from the mother plant. Soon, the leaf stalk ceases growing and the embryo plant rests on soil where it takes root.
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The sundew’s seeds are black and elliptical. Sundews living in the northern parts of the world or in mountainous climes form hibernacula to survive the winter. A hibernaculum is a small, tight, spherical cluster of budlike young leaves that are hairy in appearance. The butterwort also shares this feature. Similarly, the hangman tree’s taproot, which allows it to survive the winter, may be a further modification on the use of the hibernaculum.
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The trapping season for carnivorous plants generally runs from spring until the middle of autumn and the winter dieback. Trapping methods among the carnivorous plants fall into either active or passive traps. Among the active traps are the beartrap variety, used by the Venus’ flytrap and the waterwheel plant, and the trapdoor of the bladderwort. Less complex are the passive traps used by sundews, butterworts, and rainbow plants; these plants secrete a type of mucilage to form a sticky ‘flypaper’ trap to ensnare their victims. The simplest trap is the passive pitfall used by the many species of pitcher plants.
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The traps themselves are actually leaves that have been so modified by evolution that they are now barely recognizable as leaves. All the carnivorous plants, except for primitive species of pitcher plant, have developed digestive glands within these leaves. These glands secrete a mild enzyme to aid the breakdown and absorption of nutrients from the plant’s victims. In the case of the flypaper variety of carnivorous plants, the leaves have also developed glands to produce and secrete the mucilage used in the trap.
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How did these traps evolve, and how do they work? All plants have tropisms - reactions to particular stimuli - that help them find water, light, and nutrients; tropisms also help the plants avoid noxious substances and conditions. If seeking or avoiding something, a plant can control the growth of its cells and alter the direction of such growth by increasing and decreasing cell growth on either side of the plant. For example, the mimosa plant can fold up its leaves whenever certain insects approach with the intent of eating the leaves. For most carnivorous plants, this controlled cell growth is accelerated to the point where the plant’s movements are faster than the eye can track.
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In the case of the Venus. flytrap, prey is attracted to the plant’s trap either by the red color on the inside of the trap-leafs lobes (which resembles raw meat) or by the scent of nectar produced by glands along the edge of each lobe. (All carnivorous plants have nectar-producing abilities and, except for the waterwheel and the bladderwort, use scent as a lure.) Within the trap are six trigger hairs arranged in triangular groups of three on each lobe. The victim must brush two of the trigger hairs or one trigger hair twice in order for the plant to react. The first brushing of a trigger hair causes an electrical impulse to be stored in the leaf tissue, readying the trap. The second brushing sends a second impulse that causes the outer cells of the lobes to grow an additional 25%, thus causing the trap to close.
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This growth spurt is very rapid; closure time for flytraps has been clocked at one twentieth of a second. During cool weather, when the plant’s reactions have slowed, the spikes that fringe the edges of the leaf lobes help contain the prey until the trap is fully closed.3 When fully closed, cell growth in the lobes continues, forcing the lobes together, pushing the air out of the trap, and squeezing the prey. To open, the inner cells of the leaf lobes grow an additional 25%, thus forcing the lobes to move apart from each other. The traps are capable of opening and closing several times a day; thus, rapid growth of individual traps is possible.
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Venus’ flytraps can survive periodic flooding, when their traps catch food in the form of insect larvae, tadpoles, and the like. A good-sized meal for the plant results in an overall growth spurt. The Venus’ flytrap is capable of distinguishing between edible and inedible objects placed within its traps; it is also able to judge the size of objects so that it doesn’t waste time on puny prey or things which it can’t eat (as is also true for the giant Venus. flytrap).
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The same principle of controlled cell growth permits the sundew to curl its armlike leaves around insects held helpless in the plant’s glue. It also allows the butterwort to curl the edges of its trapping leaves to form a cup to hold digestive juices used for drowning prey that succumbs to the lure of its sticky surfaces.
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Sundews, butterworts, and rainbow plants, by utilizing their mucilaginous surfaces to trap insect prey, are merely taking defenses used by plants a step further. Consider the South African roridgula, which has developed a carpet of sticky hairs over its stems and leaves as a defense against insect pests. The roridgula has also developed a symbiotic relationship with species of ambush bugs and spiders. Both of these creatures make their homes among the sticky strands of the plant and feast on insects caught by the glue.4
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The pitcher plant may have been the first carnivorous plant to evolve because its method of catching prey is the simplest of all: a pitfall trap formed by the plant’s leaves, which have grown together so that water collects in the bottom of the pitcher and drowns whatever falls in. An intermediate stage in the evolution of the pitcher plant can be seen in the common teasel plant, whose cuplike leaves grow together around the stem, allowing water to collect and form miniature pools that protect the plant against insect pests crawling up the stem. The pitcher plant took this means of defense and evolved it into a means of trapping prey.
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The more primitive species of pitcher plant rely on natural decay to break victims down into necessary nutrients for the plants to absorb. Other species of pitcher plant have developed digestive glands and wetting agents in the water inside the trap which help waterlog the victim and hasten drowning. There is some speculation among botanists that the nectar of the plant may contain a narcotic; this increases the likelihood that exploring insects fall in due to drunkenness.5
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Fantasy carnivorous plants (i.e., the hangman tree and others) are higher on the evolutionary ladder than the common species of such plants. This higher evolutionary status may range from simple giantism without drastic departures from the original plant (as in the case of the giant Venus. flytrap) to improvements on the original plant design (as in the case of the giant pitcher plant which, in addition to giantism and heightened tropic senses, has great, long tendrils for snaring and drawing prey into its stomach).6
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The carnivorous plants in the AD&D game world have further developed alternative methods of catching prey, as is evidenced by the tri-flower frond, the man-trap, and the bloodthorn. The triflower frond and the man-trap use their pollen as both a lure and a drug to entice and kill their victims, much as actual carnivorous plants utilize their nectar as an insect lure (and as the pitcher plant might use narcotic nectar).
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The bloodthorn has taken two characteristics of plants - thorns as a means of defense, and the principles of capillary attraction (which enable plants to feed themselves) - and has combined these into a unique means of attacking prey and feeding on it. Capillary attraction is the tendency for liquids confined in small tubes to rise up through the tubes as a result of surface tension. An example of capillary attraction is shown by placing a straw in a glass of water. Surface tension draws the water up the straw from the open bottom until a balance is achieved and the water ceases to rise. If straws of different diameters are placed in the same glass, the water will rise higher in narrower straws than in wide ones because of the differences in surface tension.
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Two types of tissues are involved in a plant’s use of capillary action and attraction: xylem and phloem. Xylem is the woody tissue that provides support for the plant, much in the same way that skeletons provide support for animal bodies. The cells in xylem, through capillary action and attraction, absorb the water and minerals taken in by the roots and move them up to the leaves for photosynthesis. This process in turn creates the food material that is transported throughout the plant by the phloem tissues through capillary action and attraction.
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In the bloodthorn, the phloem tissues connect directly with the plant’s hollow thorns. When a successful strike is made by the plant, the liquid blood of the victim rises into the narrow opening of each thorn tip and is absorbed by the phloem cells. By devising a method of feeding directly on the already-dissolved nutrients in its victims. life fluids, the bloodthorn has bypassed the need to develop digestive glands like other carnivorous plants.
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Some of the species of fantasy carnivorous plants have evolved forms of vegetable musculature similar to octopus tentacles (the giant pitcher plant being the most obvious example). This has allowed some plants, such as the giant sundew, to become mobile and so increase their chances of survival by allowing movement from one location to another as an environment becomes unsuitable for them.
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The development of musculature in fantasy carnivorous plants presumes also the possible development of some form of nervous system. An increasingly complex nervous system allows the evolution of intelligence, such as typified by the semi-intelligent giant sundew. Intelligence is a survival trait; the greater a plant’s intelligence, the greater its chance of continuing as a species. In AD&D game terms, such intelligence may rise high enough to permit the development of a moral sense and an alignment other than the neutrality typical of lower animals and plants.
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Both the hangman tree and the black willow are, at present, the only carnivorous plants in the AD&D game universe that deviate from an absolutely neutral alignment.7 The tendency of both towards evil in their alignments may be due to the low level of intelligence ascribed to the plants (the black willow, though capable of possessing greater than average intelligence, does not use this intelligence to the best advantage). Creatures of low intelligence that do not possess the wisdom to control their impulsive actions tend to act to satisfy their immediate desires without consideration for others. This lack of control can also hinder the efficient application of high intelligence.8
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The hangman tree has sufficient intelligence to learn and speak the common tongue . albeit haltingly (a feat the black willow hasn’t yet accomplished).9 Nevertheless, the deception abilities of both the hangman tree and the black willow are limited, with the hangman tree using hallucinatory perfume and the black willow using its aura of drowsiness). Neither plant uses even small treasure items as a lure, possibly because they themselves are uninterested in gold and the like. Even the killer mimic is smart enough to realize it has a better chance of attracting prey by disguising itself as some valuable object.
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It is fortunate for adventurers that these trees, the highest forms of carnivorous plant presently known, are more cunning than clever; if it were otherwise, PCs might encounter these trees more often and more to their detriment. However, adventurers should be concerned by the hangman tree’s development of magic resistance as a survival trait. If the trend towards greater intelligence in fantasy carnivorous plants continues, future species of carnivorous plants may very well use spells to hunt their prey.
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Footnotes
1. Carnivorous plants, because they have retained their abilities to photosynthesize, can live a ‘meatless’ existence; during these periods, however, their growth is slower than usual and may even be stunted. As a result, a carnivorous plant that has gone through some lean times may be only one-half to three-fourths normal size, with corresponding reductions in hit dice and damage.
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2. AD&D game carnivorous plants may still be capable of reproducing this way through pollination via species of giant bees, wasps, and so forth, as well as via the normal smaller species of these insects. Seeds from carnivorous plants, though, are best harvested in the fall season. After the plant is destroyed, PCs may gather up to 2-20 seeds in perfect condition, undamaged by the battle. If PCs employ nonpoisonous means to subdue a plant peacefully, the number of useable seeds recoverable may be doubled.
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3. In issue #89’s version of the Venus’ flytrap, the ‘teeth’ that fringe the edges of the lobes act to trigger the trap and prevent the prey from escaping before the trap is fully closed.
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4. Such a symbiotic relationship might exist between the AD&D game’s giant sundew (or other sessile carnivorous plants) and certain large insects or other creatures. These creatures may lure prey within the plant’s reach in exchange for scraps from the plant’s feeding; they may also find a safe home with immunity from the plant’s attack.
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5. The giant pitcher plant may have retained the narcotic qualities of its nectar. Thus, those characters that fall prey to the plant may become unconscious from the fumes inside the plant’s stomach. Characters should save vs. poison each round they are inside the plant. Failure results in unconsciousness lasting until the victim is either consumed or removed from the plant. In the latter case, the victim remains unconscious for an additional 1-6 turns. After regaining consciousness, the victim suffers slowed reactions for 3d10 rounds, with all attacks, defenses, reactions, and dexterity bonuses at -1.
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6. Popular belief has it that certain real species of pitcher plants are able to close off the mouths of their traps with their hooded leaves, preventing victims from escaping. The giant pitcher plant may have evolved the ability to actually seal off its mouth opening. Characters who attempt to open these coverings must be able to brace themselves and successfully roll their bend bars/lift gates chances.
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7. The zygom also deviates from absolute neutrality due to its evil nature, although it cannot be rated on a scale of intelligence. The zygom has been excluded from discussion for the latter reason. Furthermore, while the Monster Manual II states that the zygom prefers living flesh and blood for sustenance, it does not actually hunt prey. Instead, it lives a parasitic existence on any host body it finds.
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8. Since both the black willow and the hangman tree are members of the plant kingdom, perhaps they cannot truly be judged by human standards. They prey on animals (including humans) for food just as many animals prey on plants for food. (Such relativistic hairsplitting is best left to philosophers and sages.)
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9. One of the knotlike protuberances on the hangman tree which usually serves as a sensory organ for the tree may have evolved a primitive vocal apparatus. This may also explain the halting nature of the plant’s speech, as the tree draws in and stores air in the knot, later expelling it in brief blasts of vocalization.

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