Question
What is in the little powder packet you get with cut flowers and does it really make the flowers last longer?  What else can you use as a substitute when you change the water a few days later?  

Answer
the packet is flower food. when that runs out i usually use lemonade instead of water in the vase.
the other ideas are, bleach, just a touch, this strengthens the stems and the flower head. the other substitute is a teaspoon of sugar.

hope this helps