Most plants make their own food using photosynthesis. We call them autotrophic. But there are a small number of plants that lack the ability to feed themselves due to the absence of chlorophyll. Heterotrophic plants must rely on other organisms to supply them with food.
Of the approximately 400,000 species of plants on earth, there are only around 3,000 that do not convert the sun’s energy into food. Heterotrophic plants tend to stand out because of their lack of green pigment. Many also have the look of a succulent.
Indian pipe (Monotropa uniflora) is an example of a heterotrophic plant. This perennial wildflower is from the same family that includes blueberries and rhododendrons (both autotrophic plants). Its strange translucent white palor make Indian pipe unmistakeable and for this reason it often goes by another common name, ghost plant. Since Indian pipe does not depend on light, it can grow even at night.
Next, let’s look at the host of the Indian pipe and the way heterotrophic plants in general function.
As with a vast majority of heterotrophic plants, Indian pipe has a parasitic relationship with its host. This means that it takes something it needs from the host, but does not provide anything beneficial to the host. Furthermore, it harms or hinders the host in some way.
Fungi are good at absorbing things from the surrounding soil. They can create extensive underground networks of root-like growths called hyphae.
The Indian pipe attaches to and taps into this network of the fungi to draw out nitrogen, phosphorus and other minerals that it needs.
But wait. Something is awry here. Although fungi are adept at absorbing minerals from soil and other substrates, they are incapable of synthesizing their own food. In other words, they are heterotrophic just like the Indian pipe (fungi are not plants).
So how do fungi eat? Fungi are decomposers (saprotrophs). They are able to break down organic matter. This is one reason they are so crucial to any ecosystem. Without this service, dead plant and animal material would pile up and nutrients would not make their way into the soil. Decomposers break down and absorb nutrients. This is how some fungi feed.
But other fungi have devised an entirely different way to get their sustenance. They colonize the roots of an autotrophic plant and form a mutualistic bond. The fungi depend on the plant to provide food from the sun’s energy in the form of carbohydrates.
In return, the vascular plant receives minerals that it needs in larger quantities than it could by gathering them directly from the soil, due to the efficient network of hyphae that the fungi has essentially made into an extension of the plants own root system.
This symbiotic relationship between fungi and autotrophic plant roots is known as mycorrhiza (see Mutualism Between Fungi & Plant Roots). This relationship is said to be so successful that over 90% of all land plants are involved in some form of mutualistic relationship with mycorrhizal fungi.
So what does this mean once we add the heterotrophic plant back into the equation? First, it explains where the Indian pipe gets its carbohydrates which allow it to grow and reproduce.
The leaves of the autotrophic plant convert sunlight into chemical energy. This chemical energy is transported through the plant’s vascular system to all parts of the plant, including the roots to fuel metabolism at a cellular level.
Some of the energy is transferred to the mycorrhizal fungi in exchange for nutrients and access to a supercharged root system.
The Indian pipe then takes some of the energy that has been relayed from the autotrophic plant to the fungi, as well as water and nutrients that the fungi have absorbed from the surrounding soil.
Approximately 400 other species of heterotrophic plants reside on mycorrhizal fungi. In the case of Indian pipe (as well as many others), they tend to grow in moist shaded forests, because they live on fungi that colonize mature cone-bearing trees.
Although Indian pipe and other heterotrophic plants do steal resources from both the fungi and the autotrophic plant, I wonder how much actual damage is caused by their association. I was not able to find much information on this subject, but I suspect, when large conifers are involved and the groupings of the herotrophic plants are fairly small, which from my observation is usually the case, the impact is often minimal.
This 3-way relationship provides a good opportunity to contrast the different approaches that two types of plants employee in order to get what they need from the same fungi–one through the cooperative effort of mutualism, and the other by parasitic means.
By briefly focusing in on the reliance that exists within mycorrhiza as well as the needs of heterotrophic plants, we may get a greater sense of the countless unseen interdependencies that exist within the natural world. A delicate balance endures in a vast web of symbiosis.