Secondary meristems are specialized regions in plants that contribute to secondary growth, allowing them to increase in thickness. Unlike primary meristems, which are responsible for lengthening the plant, secondary meristems develop from existing tissues. The most common types are the cambium and phellogen, which produce new vascular tissues and protective layers, respectively. The cambium generates new layers of xylem and phloem, enhancing the plant's ability to transport water and nutrients. Meanwhile, the phellogen, or cork cambium, produces cork cells that protect the plant's outer surface. Together, these meristems play a crucial role in the overall growth and health of woody plants.