<P> Between the xylem and phloem is a meristem called the vascular cambium . This tissue divides off cells that will become additional xylem and phloem . This growth increases the girth of the plant, rather than its length . As long as the vascular cambium continues to produce new cells, the plant will continue to grow more stout . In trees and other plants that develop wood, the vascular cambium allows the expansion of vascular tissue that produces woody growth . Because this growth ruptures the epidermis of the stem, woody plants also have a cork cambium that develops among the phloem . The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss . Both the production of wood and the production of cork are forms of secondary growth . </P> <P> In leaves, the vascular bundles are located among the spongy mesophyll . The xylem is oriented toward the adaxial surface of the leaf (usually the upper side), and phloem is oriented toward the abaxial surface of the leaf . This is why aphids are typically found on the underside of the leaves rather than on the top, since the phloem transports sugars manufactured by the plant and they are closer to the lower surface . </P>

Why do large plants usually have vascular tissue