There are two kinds of thick-walled cells in phloem: fiber cells and stone cells. Fibers are often distributed in the phloem of various plants in bundles to increase their supporting ability, which is called phloem fibers. The phloem fibers are long cells with thick lignified secondary wall, and most of them disintegrate into dead cells after maturity. However, some plants, such as flax, do not die after their bast fibers mature, and the thickened secondary wall is not lignified.

Some plants have stone cells in phloem, which can be combined with fibers alone or in cell groups of different sizes. The stony cell wall is obviously thickened, lignified and diverse in shape.