I first learned about xylem and phloem back in school … a half-century ago! So, I’m due for a bit of revision … and maybe you are as well.
Xylem and phloem are conduits, part of the vascular system of higher plants: they’re designed to move water, minerals and sugars (food) from one part of the plant to another.
“Lower plants,” like mosses and liverworts, simply count on capillary action as their source of water movement, like when you dip the tip of a cloth in water and then moisture moves further up the cloth, but then they’re only a few centimeters high at most. That won’t work for taller plants. Imagine getting water to flow up to the top of a giant redwood simply by capillary action! Instead, specialized cells line up together and transfer the products up and down, as needed.
Of course, try sending one product up a tube while another comes down the same tube at the same time in the opposite direction with any speed. That wouldn’t work! So higher plants developed two types of cells: xylem and phloem.
Xylem cells do the hard work, “pulling” water and minerals up, from one cell to the next, from the roots all the way up the stem to the very top of the plant. Rich in lignin, they also form the wood of trees and shrubs. The rings you see in a tree stump are made of xylem.
Phloem cells don’t work quite the same way. Sugars, resulting from photosynthesis and thus mostly produced by the leaves, are needed by all parts of the plant, from the flowers at the top to the roots at the bottom, so they have to flow both up and down and do so by diffusion … and also by gravity (most of the sugars eventually reach the roots). Phloem cells take the sugar-rich sap to where it is needed through their perforated cell walls.
These cells are found in all parts of the plant.
I recall in school understanding the concept, but not being able to remember which was which, which was always stressful when a biology exam was coming up. I wish someone had taught me the following mnemonic: water zips up the xylem and flows down the phloem. So simple!