What is the advantage of the xylem being uppermost in the leaf vein – facing the upper epidermis?
You are right that the xylem is normally nearest to the upper surface of the leaf in a dicotyledonous leaf. However, there are often sclerenchyma fibres on the lower side of the vein (below the phloem), so there is strong tissue on the upper and lower parts of the vein. I am not convinced therefore that the position of the xylem gives extra strength. I believe the xylem is on top because in the stem, xylem is inside the stem with phloem on the outside. Therefore it is easier to join up the xylem and phloem between leaf and stem if the xylem is on top of the leaf veins.
