DENDROLOGY

Detailed explanation-1: -The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.

Detailed explanation-2: -Stems grow vertically and, similarly to roots, they have meristematic tissue on their tips, where cells are constantly dividing through mitosis, allowing the stem to elongate. This growth in length is called primary growth. Some plants can grow indefinitely for their entire lives, even for thousands of years.

Detailed explanation-3: -They grow through a combination of cell growth and cell division (mitosis). The key to plant growth is meristem, a type of plant tissue consisting of undifferentiated cells that can continue to divide and differentiate. Meristem allows plant stems and roots to grow longer (primary growth) and wider (secondary growth).