Why are the ends of poppy and poinsettia stems sealed with heat?

In the context of floral decoration, poppy and poinsettia stems are sealed with heat—by placing the tips in boiling water or over a flame for a few seconds—because they possess milky stems. This process is necessary to prevent the loss of sap and to properly condition the plant material for arrangement.1

When performing this technique, it is important to protect the foliage and flowers from the steam and flame by inserting the stems through a hole in a piece of newspaper, which is then drawn up over them.1