We're all very familiar with green plants, from the lawns, gardens, shrubs, and trees that surround us, to the vegetables we eat. In school we learned the basics of photosynthesis... that plants somehow magically grow as long as they get adequate sunlight, water, and nutrients from the medium they're growing in.
|Sunlight, water, and green plants|
The oceans are huge gardens of green plants, in the form of seaweed. And of course forests are what we usually think of when talking about terrestrial plant life.
Those plants... they all absorb sunlight and make their own food; they're all so self-sufficient. Or are they?
Many, many millions of years ago, the roots of some plants evolved the ability to bypass the normal function of absorbing water and minerals from the soil. Instead, they began to penetrate the roots of other plants and steal from them. They tapped into the xylem, those vessels that conduct water and dissolved minerals from roots to leaves. This meant they didn't have to build as large a root system of their own, they could just take advantage of another plant's existing root system.
Thus, they became parasites. They were cheating. They still photosynthesized their own carbohydrate food, since they still had green, chlorophyll-containing leaves. But life became less of a challenge once they could steal water and minerals. They're called hemiparasites, since they steal part of their needs, but not all.
|Small-flowered False Foxglove (Agalinis paupercula)|
For unknown reasons, parasitic plants disappeared, and others later appeared. That has happened at least a dozen times over the eons, according to what scientists have deduced.
But that's not the end of the story. Along the way, some plants gained the ability to not only penetrate xylem vessels for water, but also the phloem vessels that send manufactured carbohydrate sugars from leaves to roots. Then they really had it made... everything they needed was being supplied by other plants! Didn't really need sunlight much anymore. For that matter, they didn't even need leaves, since they didn't have to make food! They're called holoparasites... they depend entirely on the host plants they pilfer from.
|Bear corn (Conopholis americana)|
In addition, there are parasitic plants that help themselves to the nutrition of other plants via the above-ground stem of their host, instead of its roots.
And fungi, which are virtually everywhere around us, particularly in forests, are part of the story too. Mycorrhizal fungi connect to the roots of nearly all trees and other plants in the forest, creating a network of interconnected plant life. Nutrients and chemical signals are passed across the network. Some plants, known as myco-heterotrophs, get their carbohydrate needs from mycorrhizal fungi instead of manufacturing their own; so, it can be said they indirectly parasitize other living plants.
So, how does all this work, and what, if anything, do these cheaters contribute to "plant society"?
Well, you can find some answers in our new film, "Plants That Cheat". See what happens down there underground in the root zone, and above ground in the stems, and how all of this relates to our forests and meadows.