In what part of the stem are the vessels located?

The vessels are located in the conductive tissue of the stem – in the wood. Also present in small size around its core.
It is through the vessels that water and various nutrients flow from the roots to the leaves.
Vessels are annually formed by cambium: in spring they are thin-walled, and in autumn they are thick-walled. This affects the light-dark color of the wood and the formation of rings.