Plant growth is facilitated by specialized regions of rapidly dividing, undifferentiated cells known as meristems. While apical meristems increase height and length, lateral meristems control growth in width. This lateral expansion, called secondary growth, is particularly noticeable in woody plants. Lateral meristem activity increases the plant’s girth, providing structural support necessary for vertical growth. This widening process creates the woody trunk and thick roots characteristic of trees.
Types of Lateral Meristems
Secondary growth is coordinated by two types of lateral meristems that form cylinders of actively dividing cells running parallel to the axis of the stem and root. The first is the vascular cambium, positioned between the primary xylem and primary phloem tissues. Its primary function is to generate new vascular tissues responsible for the internal transport of water, nutrients, and sugars.
The second type is the cork cambium, also referred to as the phellogen. This meristem develops closer to the exterior of the plant, typically in the cortex region. Its main role is protective, producing a layer that eventually replaces the original epidermis as the plant’s circumference increases.
Structural Components and Location
The vascular cambium is organized as a continuous ring of meristematic cells, including two types of initials. The most abundant are the fusiform initials, which are elongated and oriented vertically (axially) within the stem. These cells are the source of the vast majority of the plant’s secondary vascular tissue, producing cells that run lengthwise, such as vessels, tracheids, and sieve tube elements.
Interspersed among the fusiform initials are ray initials, which are smaller and more cuboidal. Ray initials are oriented radially, perpendicular to the long axis of the stem. Their division results in the formation of ray parenchyma cells, which are tissues aligned horizontally. These ray cells are essential for the lateral transport of water and stored materials across the stem, connecting the inner wood with the outer bark.
The cork cambium, or phellogen, is structurally simpler, typically consisting of a single layer of radially flattened, rectangular cells. This meristematic layer is located outside the vascular cambium, often developing from living cells within the cortex or phloem. The phellogen acts as the generative layer for the protective outer covering. As the vascular cambium pushes the stem outward, the initial epidermis ruptures, prompting the activation of the phellogen to form a new, more durable outer layer.
The Process of Secondary Growth
Secondary growth begins with the activation of the vascular cambium, which forms a complete cambial ring around the stem’s circumference. This ring is created when the intrafascicular cambium (within the vascular bundles) connects with the interfascicular cambium that develops between the bundles. The cells of the cambial ring divide in a plane parallel to the stem’s surface (periclinal division), producing new cells both inwardly and outwardly.
The cells produced toward the inside differentiate into secondary xylem, which is the primary component of wood. This secondary xylem contains highly lignified cells, such as tracheids and vessel elements, which provide mechanical strength and conduct water. Concurrently, cells produced toward the outside mature into secondary phloem, responsible for transporting sugars and other organic compounds throughout the plant.
The rate of secondary xylem production often varies with the seasons, leading to the formation of visible annual growth rings. In temperate climates, growth during the spring (earlywood) produces larger cells with thinner walls due to abundant water availability. Later in the growing season, the formation of latewood involves smaller, thicker-walled cells, which appear denser and darker. The contrast between the earlywood and latewood produced in a single year defines one annual ring, which is a record of the plant’s yearly growth.
As the vascular cambium continuously adds new layers of wood and phloem, the girth of the stem increases, placing considerable stress on the outer tissues. The cork cambium responds to this expansion by generating a new outer protective layer called the periderm. The cork cambium (phellogen) divides to produce phellem (cork) cells toward the exterior and phelloderm cells toward the interior. The phellem cells are dead at maturity and often contain suberin, a waxy substance that makes the tissue waterproof and protective.
The periderm, which consists of the phellogen, phellem, and phelloderm, replaces the ruptured epidermis and cortex tissues as the new outer skin of the widening stem. All tissues located outside the vascular cambium, including the secondary phloem and the entire periderm, are collectively referred to as bark. Because the cork cambium often forms in successive layers deeper within the stem over time, the old layers of bark are continually pushed outward, cracked, and shed.