Plants grow throughout their lives; this is called indeterminate growth, and it distinguishes plants from most animals. Although certain parts of plants stop growing (e.g., leaves and flowers), others grow continuously—like roots and stems.
Indeterminate growth in plants is enabled by meristems, tissues containing undifferentiated cells—called meristematic cells. When meristematic cells divide, some daughter cells remain in the meristem, ensuring a steady supply of undifferentiated cells. Other daughter cells elongate and eventually differentiate into mature tissue.
There are two main types of plant growth: primary growth and secondary growth. Primary growth increases the length of roots and shoots, and produces leaves. Secondary growth increases the thickness of roots and shoots—but rarely leaves—in regions where primary growth has ended.
Most plant growth occurs in two types of meristems. Primary growth occurs in apical meristems, located at the tips of roots and shoots. Secondary growth occurs in lateral meristems, which run along the lengths of roots and shoots.
All vascular plants undergo primary growth, which allows roots to explore soil and shoots to access more light. Primary growth begins with the division of undifferentiated cells in the apical meristem. Daughter cells that leave the meristem partially differentiate into primary meristematic cells. Primary meristematic cells divide and elongate, fully differentiating into mature tissues and lengthening roots and shoots in the process.
The roots and shoots of woody plants undergo secondary growth in addition to primary growth. Secondary growth is enabled by two lateral meristems—the vascular cambium and the cork cambium.
The vascular cambium develops into vascular tissue—including secondary xylem (wood) and secondary phloem. The cork cambium replaces the epidermis with the sturdier periderm. The addition of these cells increases the width of roots and shoots.
