Delphinidin

Xylem tissue is an important component of the “plumbing” and storage systems of a plant and is the chief conducting tissue throughout all organs for water and minerals absorbed by the roots. Xylem consists of a combination of parenchyma cells, fibers, vessels, tracheids, and ray cells. Vessels are long tubes composed of individual cells called vessel elements that are open at each end. As each vessel element develops, the perforation plate, in some instances, can become barlike strips of wall material that extend across the openings. However, the flow of fluid through the vessels is not blocked by the strips.

Tracheids, which, like vessel elements, are dead at maturity and have relatively thick secondary cell walls, are tapered at each end, the ends overlapping with those of other tracheids. Tracheids have no openings similar to those of vessels, but there are usually pairs of pits present wherever two tracheids are in contact with one another. Pits are areas in which no secondary wall material has been deposited, they allow water to pass from cell to cell. In some plants, pit pairs function in regulating the passage of water between adjacent cells.

In cone-bearing trees and certain other non-flowering plants, the xylem is composed almost entirely of tracheids. The walls of many tracheids, as well as vessel elements, have spiral thickenings on them that are easily seen with the light microscope. Most conduction through xylem is upward, but some is lateral (sideways). The lateral conduction takes place in the rays. Ray cells, which also function in food storage, are actually long-lived parenchyma cells that are produced in horizontal rows by special ray initials of the vascular cambium. In woody plants, the rays radiate out from the center of stems and roots like the spokes of a wheel.