These cells began to devise, then developed into aloin cells through enlargement of volume and vacuolization with the differentiation of metaphloem and metaxylem.
Containing the sugars generated in leaves during photosynthesis, phloem sap is thick, like honey, and flows down the plant's phloem tissue to distribute sugar throughout the tree.
So the xylem cells are transporting nutrients from the roots to the leaf, and the phloem are taking the freshly made sugar from the leaf to the rest of the plant.
Here at the tree's base, there are more of these particles in one tissue than the other, so the water from the phloem sap is absorbed into the xylem to correct the balance.