The phloem tissue of vascular plants is primarily composed of sieve elements and companion cells. these components allow the tissue to function in a very specialized way. sieve elements are arranged in long, cylindrical patterns that form a tube of cells. the cell membranes of these sieve elements allow molecules to pass between the cells of the tube. companion cells are responsible for taking up certain molecules made in the plant's leaves through active transport and moving them into the tubes made by sieve elements. based on the composition of phloem tissue, it is most likely used by the plant for