The Plants that Eat Animals

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When thinking of the food chain, most of us think of animals eating other animals or animals eating plants. However, some rare plants eat animals; these carnivorous plants actually attract, trap, and digest prey. This fascinating behavior has captivated botanists for centuries and made carnivorous plants commonly sold houseplants.

Although sometimes debated, carnivorous plants are usually defined as plants that attract, trap, kill, and absorb nutrients from prey. Nearly all plants derive nutrients from animals indirectly, such as up-taking animal waste; on the other hand, the defining two traits of carnivorous plants are more active: trapping and digesting. To attract prey, almost all "traps" absorb ultraviolet (UV) light, which flowers commonly use to lure insects such as honeybees and flies. Many also produce sweet-smelling nectars. Once an insect has fallen for the trap, inward-pointing hairs on the trap prevent escape. The ensnared insect is digested and absorbed. While many plants secrete digestive enzymes and nearly all absorb molecules on their surfaces, these characteristics are exaggerated and spatially restricted in carnivorous plants; carnivorous plants have extensive, localized digestive apparatuses. These plants digest diverse prey by secreting, or having symbiotic bacteria secrete, a variety of proteases, enzymes that break down proteins.

This carnivorous plant, the sundew Drosera capensis, lures and ensnares its prey with these glistening, sticky droplets coating its leaves.

While carnivory is a rather intuitive way to get food (after all, most of us eat meat), what may be less easily understood is why some plants became carnivorous and others did not. Environmental factors are central to solving this puzzle. Carnivorous plants grow in infertile areas, with little nutrients in the soil or water. Consequently, they have weak or nonexistent root systems and depend on other sources for nutrients, such as sunlight and insects abundant in wet environments. Carnivorous plants without prey to catch still grow, but such starvation can inhibit special functions like flowering. Plant carnivory is such a successful strategy that it probably evolved about six times, creating widespread genera and families by the beginning of the Tertiary period (right after the dinosaurs' demise). Now a diverse group of more than a dozen genera and 600 plant species are considered carnivorous. The many different forms that plant carnivory embodies greatly delayed botanists from accepting all variants as true carnivores.

The most recognizable carnivorous plant, the Venus flytrap (Dionaea muscipula), took some time to have its carnivorous nature be accepted. The Venus flytrap is native to North and South Carolina. Despite many botanists quickly proclaiming it to be an insect-killing machine in studies published in the late 1700s, debate lingered for decades. It struck up much public interest; in 1787 Thomas Jefferson himself collected and distributed flytraps throughout South Carolina. Flytraps are only native to this state and North Carolina. It was found that three "hairs" inside the sides of each trap control snapping; when stimulated, the trap (modified leaves) snaps shut on prey within a fraction of a second (much like in animal muscles when they contract). The trap then digests prey over the next couple weeks and will capture prey multiple times before becoming unresponsive to touch. And don't let their name fool you; flytraps catch not only flies, but other insects and spiders unfortunate enough to wander into the traps. This inspired Howard Ashman's musical classic "Little Shop of Horrors," but rest assured; these plants don't eat humans.

In the same family as the Venus flytrap (Droseraceae), sundews comprise a very different group of carnivorous plants. Sundews secrete glistening drops of a viscous, sticky substance, called mucilage, through glands on "heads" all over their leaf surfaces. This appearance gave them the genus name Drosera, Greek for "dewy." Globally distributed, Drosera were the first carnivorous plants described. First inaccurately labeled a "moss" in 1554, by the late 1700s several botanists reported seeing flies imprisoned within the leaves and noted the leaves could bend, although for decades the idea that the plant moved, let alone the idea that any plant structure could kill and absorb an insect, was heatedly debated. The debate ended when Charles Darwin published definitive experimental results in 1875; the plant does move to entangle, and kill, its prey.

Another impressive group of carnivorous plants is the "pitcher plants," which have distinctive "pitchers," cup-like formations for trapping insects. These plants' predecessors likely had slightly cupped leaves that trapped enough water so that when an insect landed, it would be trapped by surface tension, drowned, and digested by bacteria. Nutrients released from the decayed insects were absorbed by the plant leaves, creating an evolutionary preference for cup-like leaves. Modern-day pitchers have several features that enhance trapping: smooth, vertical surfaces that cause prey to slip inside; transparent regions that make it unclear where the exit is; and sweet-smelling, attractive nectars.

Clearly a successful strategy, pitchers may have evolved independently five times. They now consist of two major families: Sarraceniaceae, the marsh and bog dwellers of North and South America, and Nepenthaceae, residents of the tropics of Southeast Asia, Australia, India, and Madagascar. The first known sketch of a Sarraceniaceae traveled from North America to Europe to be published in 1601. Botanists noticed insects trapped inside the pitchers in the 1700s, but thought these were simply insects hiding. In 1791, a botanist realized these plants both catch and kill insects, but only decades later did research show that the plants receive nutrients from the dead insects; it was hard to stomach that these pitchers somehow function like the human digestive system. Sarracenia, the most commonly cultured genus, is relatively hardy and often sold as houseplants. Another popularized Sarraceniaceae family member is Darlingtonia californica, the cobra lily.

The name of the single Nepenthaceae genus, Nepenthes, is an allusion to Homer's "Odyssey" when Helen mixes wine with the drug nepenthe; some pitchers resemble a Greek drinking horn. Just like Sarraceniaceae, the idea that these plants were carnivorous faced much resistance; it was not accepted until the late 1800s, even though monstrous specimens have now been found to have caught animals as large as frogs.

Aside from snapping down on prey or making a pitfall, another fascinating method of plant carnivory evolved: vacuum-suction power. "Bladder traps" are exclusive to the genus Utricularia, or "bladderworts." These terrestrial and aquatic plants are common throughout much of the world. Originally thought to be a floatation device, only in the late 1800s did a botanist find that these rootless plants' "bladders" suck insects inside the plant by a vacuum and then digest the food. The bladders are so sensitive that even the touch of a small water microbe, such as Daphnia, triggers the vacuum action.

Although flytraps, sundews, and pitcher plants make up the main groups of carnivorous plants, many other smaller groups and borderline carnivores exist. Corkscrew plants (genus Genlisea) have underground traps, while some carnivorous bromeliads use cup-like traps, such as Brocchinia reducta. Other plants have sticky leaves similar to sundews, including butterworts (genus Pinguicula) and the Portuguese Sundew (Drosophyllum lusitanicum). There are many borderline carnivores, such as the genus Roridula; it traps insects, but, instead of getting nutrients from them, it receives food from the droppings of the insects that eat the trapped bugs. There is an astounding diversity of carnivorous plants, but many species are on the verge of extinction. In order to preserve these fascinating organisms, significant conservation efforts are needed.

For more on carnivorous plants, see Wikipedia's "Carnivorous Plant",

Barry Rice's "The Carnivorous Plant FAQ", and B. E. Juniper, R. J. Robins, and D. M. Joel's The Carnivorous Plants.

Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at science@independent.com.

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