根和茎的初生生长与次生生长

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 groun