Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Pitcher plant. © USDA ARS Photo Unit

Pitcher plant.
© USDA ARS Photo Unit

August 2009

Meat eaters with roots and leaves

Four genera of Wisconsin plants scarf up bugs to meet their nutritional needs.

Teresa A. Golembiewski

Carnivorous plants, as their name implies, are plants that eat meat! This adaptation enables them to thrive in low-nutrient environments. Nearly all of Wisconsin's carnivorous plant species inhabit sphagnum bogs.

Such bogs are northern peaty wetlands and if you visit one, you will be particularly impressed that it forms a bouncy mat of sphagnum moss and other plants. This mat is alive on top, but underneath lie several feet of barely decomposed organic matter, primarily dead plants. Sphagnum bogs are typically acidic, anaerobic and cold. These conditions discourage bacteria from fulfilling their role as decomposers. Plants and animals that die in a bog thus remain largely intact and their component nutrients donít break down, so they are unavailable for plant uptake. A carnivorous plant's ability to obtain essential nutrients (primarily nitrogen) from its insect prey allows it to thrive in this challenging environment.

Worldwide, most carnivorous plants grow in acidic, nutrient-poor, freshwater wetlands. The primary exceptions are tropical pitcher plants that grow in the nutrient-poor forests of Southeast Asia. All plants (think of your garden) need sunlight, water and fertilizer (nutrients) to grow. For every plant, the absence of one of those factors limits its growth. Since carnivorous plants most often subsist in areas with adequate sunshine and with abundant moisture, the limiting factor is nutrients.

Carnivorous plants occur on all continents except Antarctica. There are nearly 700 species in about 20 genera; Wisconsin is home to 14 species in four genera.

Sundews, butterworts, pitcher plants and bladderworts are the more common and widespread genera, while the Venus flytrap (not a Wisconsinite) is the best known. Most of us have poked a pencil into a Venus flytrap at some time in our youth, and are familiar with its bear trap-like leaves, but thatís just one of many techniques these plants use for capturing and dispatching prey. The carnivorous plants found in Wisconsin use strategies that more closely resemble flypaper, a pitfall trap or a spring-loaded vacuum cleaner. Let's have a look at these fascinating species!

Sundews

Sundews are closely related to the Venus flytrap and belong to the same family. Every bit as showy, Wisconsin's sundews are glistening jeweled rosettes. Their leaves are flattened green pads that sport bright red-stalked glands coated with shiny sticky liquid. Sundews capture their prey using the flypaper technique: small insects are attracted by the sundew's bright red coloration and sweet nectar, and become mired in its goo. As the insect struggles, the stalked glands further entrap and smother the prey. The insect dies of exhaustion or suffocation right on the leaf. Glands on the leaf surface produce acids and enzymes to break down the prey's protein. The digested products are then absorbed through the leaf surface. Any indigestible material, such as wings or exoskeleton, is later blown away by winds or washed away by rains.

Spoon-leaved sundew. © James Henderson/Gulf South Research Corp.
Spoon-leaved sundew.
© James Henderson/Gulf South Research Corp.

With more than 180 species, sundews account for one quarter of the world's carnivorous plant species. Australia, southern Africa and South America are major centers of sundew biodiversity. Four species occur in Wisconsin: two common and two rare. They are most easily told apart by their leaf shape.

The round-leaved sundew (Drosera rotundifolia) has the shortest, squattest and roundest leaves. It is widespread, occurring throughout the northeastern and western states as well as in Canada, Asia, Europe and possibly New Guinea. The spoon-leaved sundew (D. intermedia) has longer, spoon-shaped leaves and is also found throughout the eastern states, South America, the Caribbean and Europe. The round-leaved and spoon-leaved sundews are found in suitable wetlands throughout much of Wisconsin.

The English sundew (D. anglica) has the next longest and narrowest leaves. It is threatened in Wisconsin, where it reaches the southern limit of its range. Fortunately, the English sundew is not in danger worldwide, and is found in the Pacific Northwest, Hawaii, Europe and Asia. In Wisconsin, it occurs at the edges of a few acidic sphagnum bogs in Ashland and Bayfield counties.

The linear-leaved sundew (D. linearis), has the longest, narrowest leaves of the Wisconsin sundew species. It is also listed as threatened here and is truly rare. It is restricted mainly to the alkaline marl peatlands of the Great Lakes region of North America. Surprisingly, small populations are also found far to the west, in Montana and Canada. In Wisconsin, small colonies are found in Ashland and Ozaukee counties.

Butterwort

The butterwort, perhaps, looks least like a carnivorous plant. Its leaves are arranged in a rosette pressed flat against the ground. Especially when sporting its showy flowers, a butterwort looks more like a pallid African violet than a sinister meat eater. Take heed, however. Audrey of "Little Shop of Horrors" fame was a cross between a butterwort and a Venus flytrap!

Speaking of Audrey, it is interesting to note that she is most often depicted in theater and film as eating with her flower. In real life, the flowers are not involved in carnage; their sole role is in reproduction. It is a carnivorous plant's leaves that do the munching.

Butterwort. © Emmet J. Judziewicz
Butterwort.
© Emmet J. Judziewicz

It's also interesting to think about how carnivorous plants take in food compared to how people ingest. We derive both energy and nutrients from the food we eat. Since carnivorous plants are green, they get their energy through photosynthesis as do all green plants. They derive only select nutrients from the insects that they eat.

The butterwort is so named because its greasy leaves have the slick feel of cool butter. Although not closely related to the sundew, the butterwort also uses the same flypaper technique to secure its prey. However, its glands rest right on the leaf's surface rather than on stalks like the sundew. Also, the butterwort does not generally employ color or sugars to entice its prey.

Wisconsin has only one butterwort, Pinguicula vulgaris, that is found primarily on cool, sandstone cliffs on the Apostle Islands. The plant reaches the southern limit of its range at our northern border, where it is listed as endangered. North of us, its range extends from coast to coast and there are about 100 species of butterwort worldwide. The southeastern United States, Mexico, Cuba and northern Europe are major centers of butterwort diversity.

Pitcher plants

The purple pitcher plant (Sarracenia purpurea) is Wisconsin's largest and showiest carnivorous plant. Its leaves form into pitchers that have a widely winged edge and a flaring hood. The leaves can be a foot long and form a crowded cluster. The flower is large and maroon and is on a stalk that can tower to two feet tall.

Pitcher plant. © James Henderson
Pitcher plant.
© James Henderson

The pitcher plant uses a pitfall technique to capture its prey. Its pitcher-shaped leaves hold liquid, primarily rainwater. The hood, the leaf part above the trap opening, uses a bright color and sweet nectar to entice insects to land. The inner wall of the hood is covered with hairs that point downwards; an insect traversing the hood can easily walk down towards the trap's opening, but not up against the forest of stiff hairs. Lower down, the inner wall of the trap is slick, providing no foothold for the hapless insect. It falls in and dies. In most pitcher plants the prey is digested by enzyme-producing glands found on the inner surfaces of the pitcher. However, the species found in Wisconsin depends on commensal organisms to break down the prey. Indigestible parts, such as wings and the exoskeleton, remain in the bottom of the pitcher through the life of that leaf.

Of the 11 species of Sarracenia, 10 only occur in the southeastern United States and only the purple pitcher plant, Sarracenia purpurea, occurs in Wisconsin, throughout the eastern third of the United States and throughout nearly all of Canada.

Bladderwort

The biggest genus of carnivorous plants in Wisconsin and worldwide is the bladderworts (Utricularia spp). With more than 220 species, bladderworts account for two-thirds of all carnivorous plant species. They are found on all continents except Antarctica and reach their greatest number of species in Australia and the tropics.

The Wisconsin bladderworts are found in damp substrates and quiet waters of wetlands, lakes and ponds. They are small, inconspicuous wispy thin, floating or creeping plants. Their tiny traps are found on filaments often just below the water surface. Each plant may have hundreds to thousands of traps.

The bladderwort trap is especially complex. Each is essentially a tiny sac with a door that opens inward. Glands inside the trap pump out the water within, flattening the sac-like trap and creating somewhat of a vacuum inside. The trap's door is surrounded by outward pointing trigger hairs. When the hairs are touched, the door swings inward and the trap rapidly inflates, sucking in both water and prey. Once inflated, the door quickly snaps shut. Water is again expelled from the trap by the internal glands while the captured prey is retained. Enzymes released inside the trap break down the prey's proteins and the plant absorbs these released nutrients.

Common bladderwort. © Sturgis McKeever
Common bladderwort.
© Sturgis McKeever

Eight bladderwort species occur in Wisconsin. The common bladderwort (Utricularia vulgaris), is found statewide, except in the southwest corner or Driftless Area, which has far fewer suitable wetlands and quiet waters. The horned bladderwort (U. cornuta) and flat-leaved bladderwort (U. intermedia) are also quite common, though they do not range as widely. The five remaining species are seen much less often. Three of these later five are listed as rare and of special concern by the DNR's Bureau of Endangered Resources. They are Wisconsin's two purple flowered species: eastern purple bladderwort (U. purpurea) and the northern bladderwort (U. resupinata), as well as the hidden-fruited bladderwort (U. geminiscapa), one of the six species having yellow flowers. The other two members are the lesser bladderwort (U. minor) and creeping bladderwort (U. gibba).

The low-nutrient, boggy habitats where carnivorous plants thrive are relatively uncommon, and in places are sensitive to human disturbance. For instance, the well-known Venus flytrap, so ubiquitous as a cultured plant in nurseries, is at risk of extinction in the wild. Its native range is quite small – originally only within 75 landward miles of Wilmington, North Carolina. Today, its rapidly-shrinking native range consists of 13 counties in North Carolina and two in South Carolina. Poaching and habitat loss are placing it, and many of our own stat'ís unique and precious plants at risk.

To learn more about carnivorous plants, visit International Carnivorous Plant Society. Don't miss the Carnivorous Plant FAQ and its discussion of carnivorous plant books.

Beyond reading about carnivorous plants, some are available as nursery stock and you can get detailed instructions for their care from reputable growers. Two such commercial growers include California Carnivores at californiacarnivores.com and Carnivorous Plant Nursery at carnivorous plant nursery. com. If you see carnivorous plants available through other local plant suppliers, check to be sure that the plants are derived from nursery propagated stock and not collected from the wild.

Biologist Teresa A. Golembiewski is Laboratory Manager at the University of Wisconsin-Whitewater. She raises carnivorous plants and give many public presentations about these fascinating plants.