|MadSci Network: Botany|
Well, I think that the answer lies in the trace element story
This page here gives a bit of abasic explanation, and I've copied the relevant bits for you below.
Plants need 17 elements for normal growth. Carbon, hydrogen, and oxygen are found in air and water. Nitrogen, potassium, magnesium, calcium, phosphorous, and sulfur are found in the soil. The latter six elements are used in relatively large amounts by the plant and are called macronutrients. There are eight other elements that are used in much smaller amounts; these are calledmicronutrients or trace elements.
The micronutrients, which are found in the soil, are iron, zinc, molybdenum, manganese, boron, copper, cobalt, and chlorine.
All 17 elements, both macronutrients and micronutrients, are essential for plant growth. Most of the nutrients that a plant needs are dissolved in water and then absorbed by the roots. Ninety-eight percent of these plant nutrients are absorbed from the soil solution and only about 2% are actually extracted from the soil particles by the root. Most of the nutrient elements are absorbed as charged ions, or pieces of molecules (which are the smallest particle of a substance that can exist and still retain the characteristics of the substance). Ions tend to be able to dissolve in water.
The majority of the micronutrients are not mobile within the plant; thus, deficiency symptoms are usually found on new growth. Their availability in the soil is highly dependent upon the pH and the presence of other ions. The proper balance between the ions present is important, as many micronutrients are antagonistic to each other (one will cancel out the effect of another). This is especially true of the heavy metals where an excess of one element may show up as a deficiency of another. If the pH is maintained at the proper level and a fertilizer which contains micronutrients is used once a year, deficiency symptoms (with the exception of iron deficiency symptoms) are rarely found on indoor plants. Many of the micronutrients are enzyme activators.
Put more simply, some of these micronutrients are needed to maintain a healthy plant, and I imagine will probably help to keep flowers fresher for longer.
Iron (Fe) Absorbed as Fe++, Fe+++. Iron deficiency: Interveinal chlorosis (discolouring between the veins) primarily on young tissue, which may become white. Fe deficiency may be found under the following conditions even if Fe is in the soil: Soil high in Ca, poorly drained soil, soil high in Mn, high pH, high P, soil high in heavy metals (Cu, Zn), oxygen deficient soils or when nematodes attack the roots. Fe should be added in the chelate form; the type of chelate needed depends upon the soil pH. Iron toxicity: Rare except on flooded soils.
Zinc (Zn) Absorbed as Zn++. Zinc excess: Appears as Fe deficiency. Interferes with Mg. Zinc deficiency: "Little leaf," reduction in size of leaves, short internodes, distorted or puckered leaf margins, interveinal chlorosis.
Copper (Cu) Absorbed as Cu++, Cu+. Copper excess: Can occur at low pH. Shows up as Fe deficiency. Copper deficiency: New growth small, misshapen, wilted. May be found in some peat soils.
Ok now for the juicy part, what is the composition of a penny?
The Composition of the Cent (commonly known as the penny)
Following is a brief chronology of the metal composition of the cent coin (penny):
So, for most of the time that a penny has been around it had a high copper content. I'm not a plant physiologist, but I would imagine that water generally is low in copper, and putting a penny in the bottom might just give sufficient copper ions to extend the life of the tulip flowers. Or, maybe the water is very high in iron, and the little bit of copper that comes off the penny helps to counteract the effects.
However, there is probably little point in doing that now unless you use pennies from before 1982. There are commercial cut-flower micronutrient mixes available. One that is available here in New Zealand is Chrysal.
Hope this helps answer you question a bit. I also hope that I am on the right track. By the way, the story about the penny is really quite interesting. I've left a few more links to the penny pages below if you are interested too.
Happy flowering from Astrid
Changes in composition
A BRIEF HISTORY OF THE U.S. CENT
What's So Special About the 1943 Copper Penny?