All cells in the plant must be supplied with materials that move through the vascular system. In very general terms, the xylem is composed primarily of non-living cells called tracheids and xylem vessels (Figure 5). Because it is non-living, the xylem is considered part of the apoplast. In addition to the xylem, cell walls and free spaces between cells are also part of the apoplast. Herbicide movement in the xylem, cell walls and free spaces is often referred to as apoplastic. The primary function of the xylem is the transport of water and nutrients from the root to the shoot.
Figure 5. Diagram of the vascular system showing primary elements of the xylem and phloem. (Image by Dusti Duffy, Tracy Sterling, Scott Nissen, and Deana Namuth)