Xanthophyll (zan´ tho fill) [Gr. xanthos: yellowish-brown + phyllon: leaf] • A yellow or orange pigment commonly found as an accessory pigment in photosynthesis, but found elsewhere as well. An oxygen-containing carotenoid.

X-linked (also called sex-linked) • A character that is coded for by a gene on the X chromosome.

Xerophyte (zee´ row fyte) [Gr. xerox: dry + phyton: plant] • A plant adapted to an environment with a limited water supply.

Xylem (zy´ lum) [Gr. xylon: wood] • In vascular plants, the woody tissue that conducts water and minerals; xylem consists, in various plants, of tracheids, vessel elements, fibers, and other highly specialized cells.