Trachea (trā′kē̇-ȧ) (in plants), the characteristic element of wood or xylem, the tracheae together constituting the tracheary tissue. A trachea is a vessel composed of a single, elongated cell or of several cells set end to end. The cells usually are of large caliber, with very heavy walls, which are variously thickened. In some cases the thickening consists of a spiral band, in others of distinct, encircling rings. Frequently the thickening of the walls leaves definite, thin spots, called dots or pits. In this way the spiral, annular and dotted ducts or vessels are formed. The tracheary tissue is chiefly concerned in the conduction of water from the roots to the leaves; and frequently it is an important or even the chief mechanical support of the body.
Vertical section of stem from the epidermis (at the
left) to the pith (at the right), the vessels or
tracheæ being recognized by their large
size and by their pits and spirals.