An embryo plant whose root and stem have started growing is called a germinating seed.

Here's a breakdown:

* Embryo: The tiny plant inside a seed.

* Germination: The process where the embryo inside the seed begins to grow and develop.

* Root and stem: The first structures to emerge during germination, the root anchoring the plant and the stem reaching for light.

So, once the root and stem have begun to grow, the seed is considered to be germinating.