Primary and Secondary Growth in Roots and Shoots

As plants grow, their roots and shoots increase in length through primary growth. In woody plants, they also increase in thickness through secondary growth.

In shoots, primary growth begins at the shoot apical meristem. Here, undifferentiated cells divide continuously. Some of these cells then differentiate into three primary meristems: the protoderm, ground meristem, and procambium.

These primary meristems form the major tissue systems of the plant.The protoderm forms the epidermis, which protects the plant surface.The ground meristem produces ground tissues, including the cortex and pith, which help with storage and support. The procambium develops into primary vascular tissues—the primary xylem and primary phloem—which transport water, minerals, and sugars.

In roots, primary growth happens in three overlapping regions called the zones of division, elongation, and differentiation.

The zone of division contains the root apical meristem, where cells actively divide.In the zone of elongation, these cells increase in length and push the root tip deeper into the soil. In the zone of differentiation, cells mature into specialized types, such as vascular cells and root hair cells.

Although root and shoot apical meristems produce the same primary meristems, the resulting tissues differ slightly. For example, root ground tissue typically forms a cortex but may lack a central pith, which is commonly found in stems.

Primary growth also includes branching. In shoots, branches arise from axillary buds, which are located along the stem.

The growth of these buds is regulated by apical dominance, a process in which the active apical meristem suppresses nearby axillary buds through hormonal signals. When the apical meristem is removed, such as during pruning, this inhibition is lifted, allowing lateral branches to grow.

Lateral roots, by comparison, originate from the pericycle, a layer of cells located just inside the endodermis and around the vascular tissues. As they develop, lateral roots grow outward and break through the surrounding tissues.

Secondary growth increases the thickness of roots and shoots, especially in woody plants. This growth is driven by two lateral meristems: the vascular cambium and the cork cambium.

The vascular cambium produces secondary xylem, or wood, toward the inside and secondary phloem toward the outside. The cork cambium forms the periderm, producing cork cells on the outside and phelloderm on the inside. In roots, the cork cambium typically arises from the pericycle, while in shoots, it usually develops from cortical tissues.

Together, primary and secondary growth allow plants to extend into new environments while also strengthening their structure. This dual growth pattern supports plant adaptation and survival in changing environments, even though plants remain fixed in place.