Phloem are hollow tubes made of up many connected cells (sieve tubes elements). The cell walls between each of the cells are perforated into structures called sieve plates. ... The cells are dead and hollow and have very thickened cell walls that are impregnated with lignin.

Where is lignin found?

Lignin is found in the middle lamella, as well as in the secondary cell wall of xylem vessels and the fibers that strengthen plants. It is also found in epidermal and occasionally hypodermal cell walls of some plants.

Is the phloem Lignified?

His conclusions may be summed up as follows: the sieve tubes and companion cells in the stems of the species mentioned are normally lignified; the contents of the sieve tubes in the root become ligni- fied, whereas lignification of phloem parenchyma is rare, having been ob- served in only one case.