How do the differences in the cellular structure of xylem and phloem reflect their differences in ...

I hope to answer your question when stating this and if not, please just message me and I'll correct myself. Xylem serves very little other function (if any) than a "straw". Realize that water, which is predominately what flows through xylem have cohesive and adhesive properties that allow it to move against gravity (along with the fact that transpiration in plants pulls the water to where it's needed from the roots). Xylem also only transfers minerals and water that again, are pulled up through transpiration and other properties of water.
However, phloem has a two-directional flow (whereas xylem only has one). This flow brings sugars and other nutrients from the sugar source (generally the leaves and fruits of the plants if i have it right) to the sugar sink (Roots). These sugars and such cannot move without water and needs to have a proper pressure to keep moving. Water from the xylem sometimes is pushed into the phloem due to changes in pressure (water will always move from a higher pressure to a lower one) a vice versa. This exchange of fluids keeps the sugar moving to where its needed and the water moving to where it's needed. In a nutshell, the water in the phloem keeps the sugars moving to the roots where its needed and the water in the xylem is taken to the leaves and other plant parts where needed.

To keep it simple: The xylem only moves water up to the parts of the plants further from the root. It's sole purpose is almost like a straw (aside from exchanging water with the phloem at appropriate pressures) while the phloem is a tubule that must keep a good enough pressure to push the thick sugar down to the sugar sink.

If I didn't answer your question, please message me. I think I went on a tangent, but it's much easier to understand the parts and why they act like that if you know the function. Hope this helps.

Edit: Below is a website that may help you.