Question

why xylem tissue is called dead tissue

Answer 1

Non-vascular plants such as mosses don't have vascular tissue, but in all other plants xylem transports water around the plant. Xylem cells are like zombies in that they are dead when functional. ... They also have small holes called pits on their sides, which allow water to move laterally.

Answer 2

Also, they need to be hollow to allow water to pass up them; organelles get in the way especially vacuoles. So they have to be empty thus dead for that. Thirdly, the ends break down forming a continuous vessel, so if the xylem elements were alive they'd be in a bit of trouble.