MadSci Network: Botany |
The short answer bananas rot like anything else. It's a rotten world around us, and that's not a political commentary. Decomposers, primarily fungi and bacteria, are a necessary part of ecosystems. Organisms produce metabolic wastes, they shed aged, used parts, and ultimately they die. Rather than allow all these perfectly good organic molecules to pile up, decomposers are adapted to using these "wastes" for their raw materials and energy source. Decomposer fungi, molds and yeasts, have enzymes that can digest the molecules composing the banana. Molds can produce millions of spores, and a constant rain of spores sprinkles down upon us and other living organisms, but nothing much happens as long as the organism is alive. But when the organism dies, and is not using energy to maintain its cellular integrity, then the fungus has the opportunity to exploit this island of resources that it has landed upon. The banana had done what it was designed to do, sort of. The real function of a banana is to smell and taste of tropical fruity goodness to attract an animal seed disperser by appealing to their senses with smells and tastes that mean nutritious food. The banana flesh is loaded with starch and sugar to reward and please the animal, and the large, hard seeds are dispersed either discarded as the animal eats around them, or after they pass through the GI tract. Your bananas don't have large, hard seeds because they are a sterile mutant, produced by a clone of sexless plants. You can find the tiny, undeveloped abortive seeds if you look. But the banana doesn't know it has no viable seeds, no offspring, so when the fruit reaches maturity, the flesh softens by enzymatically loosening the cell wall material, starch is converted into sugar for a sweet taste, and a pleasing fruity odor is produced anyways. The banana, like other fleshy fruit, is plant tissue whose function is to be consumed, and if not eaten by a seed disperser its ripe and ready for rapid decomposition by fungi and bacteria. A more interesting question is why do food spoilage fungi make food look, smell, and feel icky, a technical term for unappetizing? Fungi are in competition with big animals like ourselves for this food resource, and if fungi aren't obnoxious when present, they'd get eaten and digested by animals who didn't notice the mold. So fungi that make their presence known and who make food look, smell, and taste bad will win the competition more often, leaving more offspring, than fungi that do not conspicuously mark their territory. This is an example of how evolution works. Our senses are either repelled by moldy food or not, and the differing reproductive success of these molds (icky food molds leave more offspring more often)have shaped their biology to win the competition for islands of food by rendering the food unappetizing. Since these islands of food are here and there, and short-lived, fungi must be capable of dispersing large numbers of offspring widely and quickly. Of course such quick growth is important because the sooner the mold announces its presence the more likely it is to win the competition by avoiding consumption. Whenever you throw out moldy food the fungus has won! There is a terrific video called "The Rotten World Around Us." It has some wonderful time lapse photography of rotting fruit.
Try the links in the MadSci Library for more information on Botany.