Definition - What does Stem mean?
In the context of wine grape cluster anatomy, the stem refers to the stalk that holds the grape cluster together. The stem can grow off of the stalks or shoots of the vine; the stem of the individual grapes is known as the pedicel while the stalk that holds the cluster together is called the bunchstem.
WineFrog explains Stem
The stem not only plays a vital role in holding the clusters together and providing nutrients to the grapes, it also imparts characteristics, most notably tannins, to the wine during pressing, crushing and fermentation. The stems grow out of the shoots, and over the course of the growing season, as the grapes grow and mature, the stem does too. In the spring, the stem is light green and quite flexible, towards the end of summer the stem will harden and change in color to brown.