As a plant grows, the shoots and roots lengthen through primary growth and, in woody plants, thicken through secondary growth.

In stems, primary growth begins when undifferentiated cells in the shoot apical meristem divide. Some progeny cells differentiate into primary meristems—the protoderm, ground meristem, and procambium.

Ultimately, these primary meristems generate mature plant tissues: dermal, ground, and vascular tissues. Specifically, the protoderm produces the epidermis, a dermal tissue. The ground meristem forms ground tissues, including pith and cortex. The procambium generates vascular tissue: primary phloem and primary xylem.

In roots, on the other hand, primary growth occurs in three overlapping zones of cells called the zones of division, elongation, and differentiation. 

The zone of cell division contains the root apical meristem. In the zone of elongation, new cells lengthen and extend the root tips. In the zone of differentiation, cells differentiate into distinct types.

The root and shoot apical meristems produce the same primary meristems, which generate the same tissue types. However, some primary tissues differ between roots and shoots. For example, root ground tissue consists mainly of cortex and lacks pith. 

Branching is also part of primary growth. Lateral shoots—or branches—grow from axillary bud meristems on the plant surface.

Hormonal signals inhibit axillary buds located near active apical meristems; this is called apical dominance. Damage or removal of the apical meristem—for instance, by a feeding animal—disrupts apical dominance. This is also how pruning encourages growth.

Unlike lateral shoots, lateral roots originate in the pericycle, located near the center of the root, and disrupt outer tissues as they emerge.

Secondary growth is similar in roots and shoots. During secondary growth, the vascular cambium and cork cambium generate the mature tissues. The vascular cambium creates secondary xylem—or wood—and secondary phloem. The cork cambium originates from the pericycle in roots and in the cortex in shoots, and produces cork cells and phelloderm. 

Primary and secondary growth contribute to the remarkable adaptability of plants, allowing them to thrive in changing environments despite their immobility.