Kristen Lindquist: The inner life of the pitcher plant

Sun, 10/21/2018 - 8:15pm

As we approach that time of year when things get a little creepy, it seems appropriate to take a closer look at one of Maine's more charismatic plants—a plant that's carnivorous! 

No, I'm not talking about anything like the murderous, human-eating plant from "Little Shop of Horrors." Our killer plant, the pitcher plant, is a more passive feeder. You won't hear it demanding to be fed or see it grabbing its prey. But this bog-dwelling plant has evolved to become very good at trapping, and eating, unsuspecting flies, ants, spiders, and moths.

Like other green plants, the pitcher plant has a sweet side; it creates sugars for itself through photosynthesis of sunlight. But a diet based solely on photosynthesis is, as Maine botanist Jill Weber says, "like drinking Coke all day. You need protein to go with your carbohydrates." Most plants get those proteins from the soils they're rooted in. But pitcher plants live in peat bogs. 

Highly acidic, low in nutrients, and almost constantly wet, bogs make challenging places for plants to thrive. So plants living there have to get more creative. 

The pitcher plant, for one, derives most of its nutrients from insects it traps inside its tubular leaves, which form little "pitchers" of rain water.

The pitcher plant isn't just waiting around for a bug to randomly fall into the water it's collected, either. The vivid red veins that run through its shiny, fleshy-looking leaves give them the look of fresh meat, especially when illuminated by sunlight. The leaves also exude an odor. An insect attracted to this potential food source (or, for a fly, this potential place to lay eggs) will land on the slippery, lipped rim of the leaf to check it out. 

Downward-facing hairs line the inner surface, enticing an insect to follow the path of least resistance toward the basin of water at the leaf's bottom. These hairs, along with sticky leaf walls, make it a struggle for the insect to climb back out again. Studies have shown that most insects do actually manage to escape from the pitcher trap. But those few that fail finally fall into the water, where they drown and become plant food. 

The leaf produces small amounts of digestive enzymes that help dissolve the soft body parts of the insect in the water. (The hard bits accumulate at the bottom of the basin.) This decomposition process is aided by bacteria and other microorganisms living inside the leaf. Larvae of mosquitoes and midges in the water also play a role in eating the drowned insects. They then pass along nutrients in their waste that are more easily absorbed and processed by the plant. By these means the pitcher plant derives the crucial nitrogen and phosphorus that it won't get from the stingy bog soil itself. And in this way the plant also hosts an entire food web within its tubular leaves. 

Weber, who graciously shared her knowledge of pitcher plants with me for this article, told me that one of her favorite things about pitcher plants is that these insectivores are apex predators. They are as crucial to their self-contained little food webs as wolves are to theirs. The plant's size and relative accessibility thus makes it the perfect model for understanding food webs on a much larger scale. 

Aaron Ellison, co-author of a recent Harvard study of the pitcher plant, says: "With pitcher plants, you can hold the whole food web in your hand. The vast number of pitcher plants in one bog provide endless opportunities for detailed experiments on how food webs work, not only in pitcher plants, but also in bigger ecosystems that are harder to manipulate, like ponds, lakes, or oceans." 

Another study conducted by the National Science Foundation discovered 35 different types of organisms living inside pitcher plants, with the various bacteria comprising just one type. How these food webs assemble "is not random," with the predator-prey interactions inside the pitchers being the critical determining component. That all this ecological drama is going on in one small bog plant seems both amazing and wonderful.  

While not Maine's only carnivorous plant, the pitcher plant is arguably the most eye-catching. Multiple pitcher-shaped leaves, with leaf openings gaping like little mouths, ring the stem base. At the end of each foot-high stem nods a single, unusual flower. The waxy-looking, yellow-green sepals and crimson petals of this flower curl inward around the round, seed-producing ovary, which is also sheltered by the "umbrella" of the large, star-shaped style. Pollen falls from anthers around the ovary into this umbrella, where it is then transferred to the stigmas (in the five points of the style) by insects gathering nectar. So while the leaves are busy below attracting insects for food, the flower above is attracting insects for sex—what more could a plant possibly want?

The only pitcher plant native to Maine, the Northern or Purple Pitcher Plant (Sarracenia purpurea), is the most common and widespread species—and also the only one adapted to cold climates. Its range covers New England, down the Atlantic coast into the Carolinas, a portion of the Gulf Coast, the Great Lakes region, parts of the Pacific Northwest and Alaska, and most of Canada. It's even the provincial flower of Newfoundland and Labrador—almost as cool as having a pine cone and tassel as your official flower. 


All photographs by Kristen Lindquist, taken in several peat bogs around Maine

 Kristen Lindquist is an amateur naturalist and published poet who lives in her hometown of Camden.

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