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Structure
Vascular system
This system consists of xylem, phloem, and vascular cambium. It can be thought of as a plant's plumbing. Xylem tubes conduct water and dissolved minerals; phloemtubes carry food such as sugars. The cambiumis a layer of meristematic tissue that separates the xylem and phloem and continuously produces new xylem and phloem cells. This new tissue is responsible for a stem's increase in girth.
The vascular cambium is important to gardeners. For example, the tissues on a grafted scion and rootstock need to line up. In addition, careless weed trimming can strip the bark off a tree, thus injuring the cambium and causing the tree to die.
The vascular systems of monocots and dicots differ (Figure 5). Although both contain xylem and phloem, these structures are arranged differently in each. In a monocot, the xylem and phloem are paired in bundles, which are dispersed throughout the stem. In a dicot, the vascular system is said to be continuous because it forms rings inside the stem. The ring of phloem is near the bark, and eventually becomes part of the bark in mature woody stems. The xylem forms the inner ring. In woody plants, it is called the sapwood and heartwood.
The difference in the vascular systems of monocots and dicots is of practical interest to gardeners because some herbicides affect only one group. For example, 2,4-D kills only plants with a continuous vascular system (dicots). Nonselective herbicides, on the other hand (e.g., glyphosate), kill plants regardless of their type of vascular system.
