Pitcher plants: their pools are cool, but not always refreshing

As I’m writing this, my weather station tells me that it feels like 95F outside, and now all I can do is think of jumping into a pool. And because in the natural world pools are not just for refreshing and drinking water, today I wanted to talk about a special natural wonder of our region, a plant that makes pools to have things jump in them… although it doesn’t always end up as a refreshment. Let’s talk about our local pitcher plants!

Pitcher plants are native to many regions of the USA, and one of these plants, the Northern or Purple Pitcher Plant (Sarracenia purpurea), is native to right here! Pitcher plants receive their name from the fascinating structure they have, which allows them to collect fluids, create a sort of pool, and use it to trap prey (usually small arthropods) that they feed upon. Although we tend to think of carnivorous plants as feeding on insects, pitcher plants have a really interesting relationship with them, since they both feed on them, while also needing them to reproduce through pollination. How does this work?

native pitcher plants in a field
The Northern or Purple Pitcher Plant is native to Maryland. Photo: A. Carlson.

Are pitcher plants really carnivorous?

First things first; let’s talk about plant carnivory. Carnivorous plants are an organism that reminds us that nature is always ready to fascinate us in the most unexpected ways. Plants usually grow by absorbing minerals from the soil and combining them with the sugars they synthesize by using the energy from sunlight and CO2. In some nutrient-poor habitats, however, conditions may be a bit too harsh to obtain sufficient minerals and food to survive. In those habitats, other feeding adaptations to supplement these low nutrient levels have evolved, allowing plants to obtain sufficient food to properly develop. The evolution of carnivory in plants is one of these strategies.

Although pitcher plants are carnivorous, this does not mean that they are a sort of plant-lion waiting there to attack and retain prey. Their ways are more intricate, and in some ways, more fascinating than those of an animal predator. As all plants, pitcher plants are not able to ingest their prey; they have no mouths, no teeth… so how do they do it?

pitcher plant
Pitchers in pitcher plants fill with rainfall and get slowly colonized by a multitude of microbes and other organisms. Photo: A. Carlson.

In pitcher plants, the pitcher (a special modified leaf) fills with a liquid formed by water (often from rainfall) and other compounds that make it really favorable for the establishment of microbial communities. Along with the secretion of some plant digestive compounds into the fluid, it is these microbes that actually digest the insects that fall into the pitcher. Once the prey is trapped in the pitcher and then digested, the plant is able to absorb the released nutrients directly through its tissues, eventually obtaining food from animal tissues, thus becoming a carnivore.

Why do insects fall into the pitcher?

Insects are often attracted to the pitcher by the many lures that the structure has. For instance, the walls of the pitcher display lines and markings that are attractive to insects, which direct them to the lid of the pitcher. At the edges of those lids there are small glands that secrete nectar, which is also mixed with some narcotic substances that make insects lose control of their bodies. Once these insects experience the effects of these substances, they lose grip and fall in the pitcher, where hairs and a slippery and narrow surface prevent them from crawling or flying out.

insects inside a pitcher plant
Many small insects fall into the pitchers, where they are digested by the pitcher microbial communities. Photo: A. Carlson.

But pitcher plants need to be pollinated too! How do they do it, if they eat insects?

Yes, pitcher plants need the service of pollinators to produce seeds and reproduce. And indeed, they also have very pretty flowers (which in Maryland bloom in May-June). So, how do they attract pollinators to their flowers instead of to the pitchers, and then, not have them fall in the pitchers by mistake? 

Studies on this are demonstrating that pitchers and flowers in pitcher plants are not active at the same time. While the plant flowers only for a very restricted time in the year, the pitchers are active most of the growing season. However, their level of activity and attraction are reduced during the flowering time. This means that pitcher plants have evolved to allow pollen transfer to happen without endangering the valuable pollinators.

pitcher plant flowers
The pitcher plant flowers are very pretty, with a special round shape and coloring. In Maryland, pitcher plants bloom in the months of May and June. Photo: B. Wheeler.

Pitcher plants are tiny ecosystems

With more and more studies done on these plants, it is now clear that the fluids in the pitchers behave really like tiny ecosystems. In some cases there are not only microbes that help the plant get their nutrients, but also other organisms that feed on these microbes. There are organisms that use the pitcher’s fluids to develop (the larvae of some mosquito species are specialized in developing in these fluids). The pitchers also are used by other arthropods to trap prey (some spiders build their webs in the pitchers).

mosquito larvae in pitcher plants
Some mosquito species, like this Maryland specimen of genus Wyeomyia, have evolved to be able to develop exclusively in the pitcher fluids of pitcher plants. Photo: K. Schultz.

I love pitcher plants and I want one in my house

It is absolutely great to be interested in carnivorous plants and pitcher plants in particular. Unfortunately, the Northern Pitcher Plant is currently classified as Imperiled in our State. Indeed, many of the habitats they prefer (bogs, peatlands) are endangered, or have been profoundly disturbed by human activity. Another additional pressure that our native pitcher plants experience is collections from the wild for trading. Indeed, the market for carnivorous plant lovers is huge, and it is cheaper for a seller to collect a plant from the wild than to grow it from seed in a nursery. If you are considering purchasing pitcher plants for growth at home, make sure that the plant you buy has not been collected from the wild.

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.

Anahí also writes an Extension Blog in Spanish! Check it out here, extensionesp.umd.edu, and please share and spread the word to your Spanish-speaking friends and colleagues in Maryland. ¡Bienvenidos a Extensión en Español!

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