34.8:
Primary and Secondary Growth in Roots and Shoots
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.
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.
Suggested Reading
Ichihashi, Yasunori, and Hirokazu Tsukaya. 2015. “Behavior of Leaf Meristems and Their Modification.” Frontiers in Plant Science 6 (January). [Source]
Pierre-Jerome, Edith, Colleen Drapek, and Philip N. Benfey. 2018. “Regulation of Division and Differentiation of Plant Stem Cells.” Annual Review of Cell and Developmental Biology 34 (1): 289–310. [Source]