Meristems and Plant Growth

Plants can grow throughout their lives, unlike most animals, which stop growing after reaching a certain age.

Plant growth begins in meristems, tissues that contain undifferentiated cells. These cells can either remain meristematic or differentiate into specialized tissues.

Vascular plants grow longer through primary growth, which lengthens roots and shoots.

Primary growth begins in the apical meristems at the tips of roots and shoots. Cells in these regions divide by mitosis. Some daughter cells remain meristematic, while others begin to differentiate into the primary meristems.

The primary meristems—the protoderm, ground meristem, and procambium—differentiate into distinct primary tissues.

The protoderm becomes dermal tissue, which forms the protective outer layers. The ground meristem develops into ground tissue, which makes up most of the body of a young plant. The procambium forms vascular tissue, including xylem and phloem.

In addition to primary growth, woody plants also undergo secondary growth. Secondary growth widens roots and stems, but it rarely affects leaves. Two lateral meristems contribute to secondary growth: the cork cambium and the vascular cambium.

The cork cambium produces cork to the outside and phelloderm to the inside, forming the periderm.

The vascular cambium forms a thin cylinder of dividing cells between the primary xylem and primary phloem. It produces secondary phloem to the outside and secondary xylem, or wood, to the inside.

In many trees, this activity produces annual growth rings, which scientists use to study climate.

Wood formed early in the growing season, usually in spring, has cells with larger diameters and thinner walls. This wood often appears lighter in color and is called earlywood. Wood formed later in the growing season, typically during summer and early autumn, has cells with smaller diameters and thicker walls. This wood often appears darker and is called latewood.

In many trees growing in seasonal climates, earlywood and latewood together form one growth ring each year.

Wider rings usually show years with better growing conditions, while narrower rings show years with more stressful conditions.

Overall, primary growth allows plants to extend their shoots in search of light and their roots to gather moisture and nutrients. Meanwhile, secondary growth provides structural support as woody plants continue to grow.