34.8: Primary and Secondary Growth in Roots and Shoots
Vascular plants, which account for over 90% of the Earth’s vegetation, all undergo primary growth—which lengthens roots and shoots. Many land plants, notably woody plants, also undergo secondary growth—which thickens roots and shoots.
Primary and secondary growth can occur simultaneously in a plant. While primary growth occurs in newer plant regions, secondary growth transpires in regions that have completed primary growth. There are overlaps and distinctions between root growth and shoot growth.
Apical meristems enable the primary growth of both roots and shoots - with primary shoot growth beginning in the shoot apical meristem and root primary growth starting in the root apical meristem.
Dividing cells in the root and shoot apical meristems differentiate into the same primary meristems—the protoderm, ground meristem, and procambium. In both roots and shoots, these primary meristems develop into the same tissue types; the protoderm, ground meristem, and procambium respectively develop into dermal, ground, and vascular tissues.
However, there are differences between the specific tissues produced in roots and shoots. In roots, the epidermis contains roots hairs, which account for most of the root’s surface area. Additionally, unlike the shoot ground tissue of eudicots—the most common flowering plants—root ground tissue is not divided into pith and cortex. Furthermore, the shoot apical meristem contains leaf primordia, which form leaves.
Compared to primary growth, secondary growth is more similar between roots and shoots. Secondary growth is enabled by two types of lateral meristems, which run along the lengths of roots and shoots.
The vascular cambium, between the primary xylem and primary phloem, creates a layer of secondary xylem (wood) and secondary phloem each year in roots and shoots.
The cork cambium, a component of bark, is located outside the vascular tissues. In both roots and shoots, it produces cork and phelloderm to form the periderm—which replaces the epidermis.