How is the internal structure of the stem related to its functions?

The main function of the stem is the transport of substances. And the internal structure of the stem is subordinated to this function.
There is a layer in the bark of the stem called the bast. It consists of conductive phloem tissue (sieve tubes). A downward flow is carried out along it, that is, the movement of organic matter from the leaves to other organs, including the root.
Wood is formed by another type of conductive tissue – xylem (vessels). It forms an upward flow, that is, the transport of water and minerals from the root.