Unlike monocots, most dicots have meristems within their stems and roots that can produce true secondary growth. This enables many dicots to grow to great heights because the increase in width supports the extra weight. In addition to showing primary growth, Figure 23–13 illustrates the pattern of secondary growth in a dicot stem.

In conifers and dicots, secondary growth takes place in meristems called the vascular cambium and cork cambium. The vascular cambium produces vascular tissues and increases the thickness of stems over time. The cork cambium produces the outer covering of stems. Similar types of cambium tissue enable roots to grow. The addition of new tissue in these cambium layers increases the thickness of stems and roots.

Growth From the Vascular Cambium In a young dicot stem, bundles of xylem and phloem are arranged in a ring. Once secondary growth begins, the vascular cambium appears as a thin, cylindrical layer of cells between clusters of vascular tissue. This new meristem forms between the xylem and phloem of each vascular bundle. Divisions in the vascular cambium give rise to new layers of xylem and phloem. As a result, the stem becomes wider. Each year, the cambium continues to produce new layers of vascular tissue, causing the stem to become thicker and thicker.

In Your Notebook List in sequence all the tissues found in a mature woody stem. Start from the center and move outward.

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