For this month's Field Notes, Susan Knight explores the ecology of Indian Pipe, spooky looking white plants that skip photosynthesis and steal sugar from their forest neighbors.
It’s October, and time to for a good spooky mystery. Have you ever been out in the woods in summer and seen Indian pipe, also known as ghost plant? These white, waxy plants are kind of creepy looking and they seem to pop up out of the ground overnight. They are about 6 inches tall, and you often see little gangs of them lurking together in the dark. Each plant has one flower, bent like a candy cane toward the ground. After the flowers are pollinated and fertilized, the little nodding pipe straightens up and the flower looks straight up into the sky. As they mature, they turn dark and hard. If you look around, you will likely see some of last year’s Indian pipes which are now black stalks topped by a woody rattle. Specters of Halloweens past.
Indian pipe, or Monotropa uniflora, are white because they have no chlorophyll and so cannot photosynthesize. Because they can’t photosynthesize, they cannot make their own sugar. They need sugar, along with nutrients usually gathered by the roots, to make all the compounds they need to grow and reproduce. How do Indian pipe, and others plants like them that lack chlorophyll survive? For years, botany and ecology students were taught that these plants were saprophytic. Saprophytes are organisms that live off of dead stuff. But, it turns out Indian pipes and other plants without chlorophyll do not live off of dead stuff, but instead, most are parasites, and live off of living plants.
But the story is complicated; Indian pipe don’t just tap into another plant and suck out the sap like a mosquito sucking blood out of a sweaty camper. Though out of sight and underground, there is vast network of fungi that exists as extremely fine threads, known as hyphae. These fungal threads are spread out like an enormous spider web and they rule an entire underground economy, essential to keeping the forest ecosystem humming. The mushrooms you see in your mushroom identification book all have these vast networks of hyphae doing all the work underground or sometimes inside dead and dying trees. And what do they do? For one thing, they are responsible for decomposing all the dead stuff on the forest floor. They recycle the nutrients out of the dead plants and make them available to living plants. The hyphae may get together to form an above ground mushroom, whose function is to make spores so that the fungus can spread itself through the world. But the hyphae are the workhorses of the fungus. It’s just like in an apple tree, where the stems, leaves and roots do all the hard work and then the apples come in and steal the show. The underground hyphae are the unsung, but critical, laborers of the fungal world.
Fungi are not photosynthetic and they must find a source for the sugar they need to live and reproduce. Many fungi tap into the roots of living plants, and these fungi are called mycorrhizae. Almost all plants have associations with mycorrhizal fungi. Some mycorrhizae and plants are picky about their partners while other plants and fungi are more promiscuous. Mycorrhizae act like extensions of the plant root system, and they are critical in helping gather water and nutrients which they pass on to the living plants. And what do the mycorrhizae get for their trouble? They get some of the sugar the plants produce from photosynthesis. It seems a fair trade and an efficient symbiosis.
So back to the Indian pipe. Remember that they, like the fungi, have no chlorophyll. They cannot make their own sugar. Mycorrhizae to the rescue! The mycorrhizae tap into the Indian pipe, and at the other end, they tap into a living plant. In this way, the mycorrhizae act as a bridge between the Indian pipe and a tree, that serves as the unwitting sugar donor. The Indian pipe is one of many plants with no chlorophyll that tap into mycorrhizae to sponge off of living plants, and are called myco-heterotrophs. The ‘myco’ part means fungus, since they are using the fungus to procure their food. ‘Heterotroph’ means they have to go get food; they cannot make it themselves. And the mycorrhizal fungus that acts as the bridge between the Indian pipe and the tree is usually a Russula fungus, many of which are edible.
Somehow, the Indian pipes have conned the mycorrhizae into giving them the sugar the mycorrhizae have taken from the tree, even though the Indian pipe delivers nothing in compensation. Why should the mycorrhizae go along with this arrangement? Why do they bother to tap into the free-loading Indian pipe when they get nothing in return? Now that is a spooky mystery, and no one seems to know.
For Field Notes, this is Susan Knight of UW-Madison’s Trout Lake Station.
Image by Arhur Meeks. http://wisflora.herbarium.wisc.edu/taxa/index.php?taxon=4275