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The periderm is a protective tissue that forms on the outer surface of woody plants. As plants grow, they undergo a process known as secondary growth, which leads to the expansion of their stems and roots. The epidermis, which is the initial outer layer, cannot stretch to accommodate this growth, so it is replaced by the periderm.

The periderm itself is composed of three layers: the phellem, or cork, which is the outermost layer and provides a waterproof and resistant barrier; the phellogen, or cork cambium, which is the stem cell layer producing new cork cells; and the phelloderm, which is the innermost layer of the periderm. Collectively, these layers contribute to preventing water loss and protecting the plant from pathogens and physical damage.

Secondary phloem is the inner layer of the plant bark which functions in the transportation of nutrients, particularly sugars produced by photosynthesis, throughout the plant. Unlike the primary phloem, which forms as part of the initial growth of the plant, secondary phloem is produced by the vascular cambium during the secondary growth of the plant.

As new layers of secondary phloem are produced, the older phloem gets crushed and becomes a part of the bark, adding to its thickness. The efficiency of nutrient transport and the health of a plant depend greatly on the functionality of the secondary phloem, making it an essential component in the survival and growth of woody plants.

The vascular cambium is a thin layer of meristematic tissue located between the secondary xylem (wood) and the secondary phloem in woody plants. It plays a critical role in secondary growth by dividing and giving rise to new cells that become either part of the secondary xylem or part of the secondary phloem.

As a lateral meristem, the vascular cambium extends along the length of the stems and roots and forms a continuous ring. It grows in circumference as the plant grows wider, which is known as radial growth. The vascular cambium is the source of thickness in stems and roots, and its activity is crucial for the plant's ability to transport water, nutrients, and sugars through the xylem and phloem, respectively.

Secondary growth refers to the process whereby woody plants increase their girth or thickness, thanks to the action of the vascular cambium and cork cambium (part of periderm). This growth pattern allows woody plants to grow taller and sturdier, supporting more weight and resisting environmental stresses such as wind and snow.

During secondary growth, the vascular cambium produces new layers of xylem and phloem, enhancing the plant’s ability to transport fluids. On the other hand, the cork cambium produces the periderm, fortifying the plant's defense and sealing off it from external threats. Together, these processes result in the characteristic thickening of the stems, branches, and roots of trees and shrubs, enabling them to become larger and live longer.