Primary plant cells are formed during primary growth, which increases the length of the plant. They are found in the apical meristems of roots and shoots, and in young, non-woody tissues like leaves and herbaceous stems. Secondary plant cells are formed during secondary growth, which increases the girth or width of the plant. They are produced by lateral meristems (vascular cambium and cork cambium) and are found in older, woody parts of stems and roots, forming tissues like wood (secondary xylem) and bark (secondary phloem and periderm).