very good info - thanks very much!

dps wrote:
It probably contains (1) sugar to feed the plant, (2) something to
acidify the water to make it more compatible with the ambient plant
juices and (3) some sort of sterilization agent to kill bacteria which
could otherwise multiply and block the passages that carry water to the
plant.

See
http://www.umass.edu/umext/floricult.../presvcut.html

This site includes a receipe for making your own cut flower preservative
with common household materials (or at least stuff readily available in
your local grocery store [e.g. non-diet Sprite or 7-up {containing both
sugar and citric acid note that I'm a fan of nested parenthetical
expressions |not all of which are relevant|} and bleach for
sterilization -- see the site for proportions]). Since requirements vary
with the flower species, the site gives a few examples.

This mixture doesn't replace roots, so the plant won't last "forever",
but it will extend vase life by as much as a week or two (depending on
plant varieties and pre-vase care).

Note that if you're cutting your own flowers, placing the flowers
immediately (within a second or two) into water will help a lot. If that
isn't practical, cutting an inch off the stem underwater will help.
Essentially, the flower is sucking water from the ground. When cut, it
sucks air into the stem, producing an air bubble (vapor lock) and
inhibiting further water takeup. Placing the stem in water immediately
minimizes the bubble and cutting the stem underwater removes it.