It is called a radicle, and is similar to the long tap root on trees. It can grow very long, so that's why we use the 16 oz cups as they give it enough room to extend. You will notice that it often has a slight curve to the tip. When planting the germinated seed, it is best to lay it on its side, so that the tip of the radicle points downward. The first green shoot will then begin to grow upward on the top side of the radicle.

Nancy