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.

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