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Mike Maddock, General Manager
2006-03-03 / Beauty in the Backyard

The Original Mystery Plant

Dr. John Nelson

Photo by Linda Lee
Photo by Linda Lee The air that we all breathe is mostly (almost 80%) nitrogen.

Nitrogen is odorless, tasteless, and colorless, and it doesn’t react much, as a gas, in our atmosphere. Nevertheless, it is extremely important. Plant and animal life would not exist anywhere in the world without it. Nitrogen is critically important in the formation of amino acids and proteins, as well as nucleic acids (which is where DNA comes from). We breathe it in all day long, but no functional nitrogen ever enters our bodies…it is only available in other chemical forms. Fortunately for us, many plant species, especially members of the bean family, are able to convert gaseous nitrogen (with the help of friendly bacteria in the soil) into ammonium and nitrates, which are readily used by living organisms. The whole process is called “nitrogen fixation.”

Plants taking ammonium and nitrates into their own systems are able to make complex nitrogen–containing molecules, and when we eat these plants, nitrogen is available to us.

Our mystery plant is a champion at turning gaseous nitrogen into useable forms. It’s an annual species, growing for only a single season. It is a common herbaceous member of the bean family, featuring bright green compound leaves, each with several pairs of narrow leaflets. The tips of the leaves are specially modified into thin tendrils which grab and hang onto other, taller plants, allowing the little herb to sprawl or clamor on top of other vegetation.

Slash pine, 
Pinus elliottii
Slash pine, Pinus elliottii A couple of flowers will be found at each leaf base. In each flower, five bright pink petals are arranged with the largest on the top, the others at the side, projecting forward. Some botanists have thought this shape looks something like a butterfly and have come up with a term for such flowers, “papilionaceous.” The mystery plant produces slender green bean pods, which at maturity turn black. The two halves of the pod, once it has split open, are prominently spiraled. A number of dark, spherical seeds are produced in each pod.

This species is native to Europe and Asia but is now widespread around the world. It has long been important as a kind of “green manure” and is commonly planted. Once plowed back into the ground, the nitrogen that has been fixed into its tissues is available to the next planted crop (cotton, corn, etc.) allowing less use of artificial nitrogen–containing fertilizers.

This little plant is now common over much of the USA and is starting to bloom now. It can be a bit weedy, but if it’s in your yard breathing some of your air, think of it as good for the soil.

Answer to last week’s mystery plant

Dr. John Nelson is the curator of the USC Herbarium.

To learn more about the Herbarium, call him at

777-8196. His department also offers free plant identification.

Photo by Linda Lee

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