34.7: Meristems and Plant Growth

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.

Unlike most animals, plants can grow throughout their lives. Plant growth is facilitated by meristems, tissues containing undifferentiated cells that can perpetually divide—akin to animal stem cells.  

All vascular plants undergo primary growth, which lengthens roots and shoots. 

Primary growth begins in apical meristems, located at the tips of stems and roots. Here, undifferentiated cells divide. Some daughter cells remain in the meristem and go on to divide again. Others partially differentiate into primary meristems.

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

The protoderm becomes dermal tissue, 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 rarely 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 is located between the primary xylem and primary phloem. Here, secondary growth generates secondary phloem to the outside and secondary xylem, also known as wood, to the inside.

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

Wood grown in the spring contains cells with larger diameters and thinner walls than wood produced later in the growing season. A year of growth, therefore, manifests as a distinct ring.

Since trees grow well in warm environments with plenty of water, thick rings indicate warm, wet years and thin rings indicate cold or dry years.

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 to accommodate the lengthening of woody plants.