Periderm—the Outer Defense Layer. The first defense line in conifer stems is the outer surface of the bark, the periderm. Inside the periderm is the cork cambium (or phellogen), a secondary meristem that produces cork tissue (phellem) outwards and secondary cortex (phelloderm) inwards.so option B.

Formation of Periderm from Phellogen

Periderm is the protective tissue that replaces the epidermis in older stems and roots of woody plants. It is formed from a meristematic tissue called phellogen or cork cambium. The phellogen is a secondary meristem that develops in the cortex, usually in the region between the epidermis and the vascular tissues.

Formation of Phellogen
The phellogen is formed from the parenchyma cells of the cortex. It originates as a result of dedifferentiation of cortical cells, which revert back to a meristematic state. The cells of the phellogen divide periclinally, meaning they divide parallel to the surface of the organ. This division leads to the formation of two daughter cells - the outer layers become phellem or cork cells, while the inner layers become phelloderm cells.

Formation of Phellem
The phellem cells are dead at maturity and have thick walls that are impervious to water and gases. These cells form a protective layer that helps prevent water loss, mechanical injury, and pathogen invasion. The phellem cells are filled with suberin, a waxy substance that further enhances their protective function.

Formation of Phelloderm
The phelloderm cells are living at maturity and function as a storage tissue. They often contain starch grains and other reserves. The phelloderm is not always present in all plants, and its development depends on the species and environmental conditions.

Continued Activity of Phellogen
The phellogen continues to divide and produce new cork cells and phelloderm cells, resulting in the increase in girth of the plant. The new cork cells are added to the outer layers of the periderm, while the older cells are pushed towards the inner layers. This process is responsible for the formation of the bark in older woody stems.

Conclusion
In conclusion, periderm is formed from the phellogen or cork cambium, which is a secondary meristem derived from the dedifferentiation of cortical cells. The phellogen produces cork cells (phellem) towards the outside and phelloderm cells towards the inside. The periderm provides protection to the underlying tissues and helps the plant adapt to its environment.