Phellogen produces cork/ phellem towards periphery and phelloderm on inside. this extrastellar growth cause pressure and rupture the cell layers present outside the pericycle .i.e. endodermis and cortex.

Answer:
The tissue outside the pericycle becomes dead due to the activity of phellogen in the dicot root.

Explanation:
Dicot roots have a distinct organization of tissues, which includes the epidermis, cortex, endodermis, pericycle, and vascular tissues. The phellogen, also known as the cork cambium, is a lateral meristem that produces cork cells on its outer side and phelloderm cells on its inner side. The activity of the phellogen leads to the formation of the periderm, which replaces the epidermis in older roots.

Role of Phellogen:
The phellogen is responsible for the formation of cork cells, which serve as a protective layer in the root. It replaces the epidermis and provides protection against desiccation, mechanical damage, and pathogens. As the phellogen produces new layers of cork cells, the older cork cells become dead and provide additional barrier properties to the root.

Cessation of Activity:
The activity of the phellogen is limited to the outermost layers of the root and gradually ceases as the root continues to grow in thickness. Once the phellogen stops its activity, the outer tissues, including the cortex, endodermis, and hypodermis, are pushed outward and eventually become dead.

Tissue outside the Pericycle:
The pericycle is a meristematic tissue located just inside the endodermis. It is responsible for the formation of lateral roots and secondary growth in roots. As the phellogen continues to produce cork cells, the tissues outside the pericycle are gradually pushed outward and become nonfunctional. These tissues, including the cortex, endodermis, and hypodermis, undergo cell death and are replaced by the layers of cork cells produced by the phellogen.

Therefore, it is the tissue outside the pericycle that becomes dead due to the activity of the phellogen in the dicot root.