meristem (n.)

"growing cellular tissues of plants, actively dividing cell tissue," 1862, formed irregularly from Greek meristos "divided, divisible" (verbal adjective from merizein "to divide, distribute," from meros "a part, a share;" from PIE root *(s)mer- (2) "to get a share of something") + ending from xylem, etc. Related: Meristematic.