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Jan 30 2013

Cellulose Conundrum

By John Nelson

Can you guess this week’s Mystery Plant? (Photo by Rick Hill).

Let’s talk about cellulose! I know you want to!

Cellulose is one of the most important organic compounds on earth, and it is made only by plants. It’s what we call a polysaccharide, in that it consists of chains of zillions of molecules of glucose, a really “simple” sugar, all strung together in a line. The individual molecules of glucose are linked together in a very important way, resulting in a strand of cellulose. (Pay attention, now!) There is a slightly different way in which these glucose molecules can be linked together…and that would give us starch. Starch is easily broken down and used as an energy source by practically any critter you can think of, but cellulose? NO WAY. Cellulose is one of the most durable molecules in nature, and it is vitally important to plants as a structural component of their cell walls. (There are a variety of microorganisms that can digest cellulose, of course. People like to think that termites enjoy eating wood…but it is actually some very special microbes inside their guts that do the actual digestion.)

Depending on where a cell is in a plant, its walls may be quite thin and flimsy, or very, very thick, and quite strong, all dependent upon the amount of cellulose (and maybe a few other things) deposited within the wall during the development of that cell. Some of the very thickest cell walls may be found on the outside of a seed, as what we call the seed coat. Which brings us to our Mystery Plant:

It’s a tropical vine, related to mimosa, and it forms huge growths in rain forests of Central America and northern South America. (The vines can be so extensive that they are sometimes called “monkey’s ladder.” Clever!) The vines produce bean pods that are the largest in the entire bean family…sometimes 5-6 feet long. Each pod contains a number of seeds. Now here is the part about the cellulose: the wall of each seed is extremely thick, and watertight. When the pods break apart, the seeds end up on the ground, sometimes falling into nearby creeks. They remain viable for years. Now during flood events or heavy rains, the seeds float around over the forest floor, stopping when the waters recede, where they will eventually sprout. But some of them float down the creek, into a river, and ultimately into a large body of water…say, the Atlantic Ocean. The seeds are thus picked up by various currents…say, the Gulf Stream, and they can float tremendous distances, far from where they originated. (The photo shows a seed that washed up on a beach in Scotland). Of course, seeds such as this have no chance of sprouting or growing, which is sort of sad. But just think what the Europeans must have thought when they found these things on their beaches long ago, knowing that these seeds must have floated there from a far distant shore, perhaps bolstering the notion of a New World out there.

Answer: “Sea heart”, “Sea bean”, Entada gigas

John Nelson is the curator of the Herbarium at the University of South Carolina, in the Department of Biological Sciences, Columbia SC 29208. As a public service, the Herbarium offers free plant identifications. For more information, visit or call 803-777-8196.

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