|MadSci Network: Botany|
All parts of a plant need sugar to live because it provides energy. Now when you cut a flower, you usually keep some of the stem with it. The stem and some parts of the flower, in addition to leaves, do absorb the sun's energy and make sucrose,which can then be transported to other areas of the plant.
Adding sugar to the water in which you place the flowers will not have any positive effect on the life of the flower. This is because roots (or in this case the base of the stem) only absorb salts and water, which are then transported to the rest of the plant. They do not absorb sugars.
In your house you have some pipes which transport hot water and others that transport cold water. Pipes in your home are equivalent to "conducting vessels" in a plant. Plants have conducting vessels which transport water and salts, and other vessels which transport sugars. In fact, adding sugar could be detrimental to flowers because it will make it harder for the flowers to absorb water and they will wilt. This effect becomes stronger as you raise the concentration of the sugar in the water.
This could be a good science experiment. If you try placing cut flowers in different concentrations of sugar from very low to high you can then time how long it will take for the flowers to die.
Incidentally, florists do not use sugar to keep flowers in good condition. They use a mixture of salts, which are sometimes given to you when you buy cut flowers.
I hope this answers your question.
David Hershey adds the following:
The sugar placed in the vase solution is used by cut flowers for energy as a substitute for the sugars normally produced by photosynthesis. Photosynthesis in cut flowers is minimal because of the low light levels in our homes and because many of the leaves are often removed. Cut flowers can absorb the sugar solution through their xylem because the base of the stem has been cut open. This is different from the normal situation where the stem is attached to the roots, and roots absorb all the water. Not all cut flowers may benefit from sugar and may be injured if the sugar concentration is too high. Use of sugar is highly recommended for cut roses and snapdragons.
Flower preservatives that dissolve in water consist of sugar and a compound(s) that acts as an acidifying agent and biocide, such as 8-hydroxyquinoline citrate. A biocide kills microbes so they don't clog the xylem and prevent water uptake by the stem. The acidfying action increases water flow through the stem. Using a floral preservative can double or triple vase life of some cut flowers.
Larson, R.A. 1992. Introduction to Floriculture. New York: Academic Press.
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