It's called two way street because the stem transfers water and minerals to the leaves in upward direction for the photosynthesis to take place by xylem.At the same time it transfers the food (glucose) from the leaves to other parts of the plant in downward direction by phloem.That's why it is called a two way street.
1 2 1
stem is called a two way street because it has two part takes water and minerals from the root and the other which takes food to each and every part of the body.