Question

Which tissue replaces primary epidermis when plant grows older

Answer 1

The tissue that replace the primary epidermis in palnts when they grow old is known as Periderm.

The periderm acts a protective layer like the epidermis this is in woody plants.
Woody stems and in some cases other stem structures produce this secondary covering known as periderm which later on when the plant grows old replaces the epidermis as a protective covering. This also happens in the roots of some plants.

Therefore, the tissue is known as Periderm

Answer 2

The epidermis of the plant is called dermal tissues or Covering tissues which covers the leaves, shoots and young stems. As the plant grows old a tissue called periderm replaces epidermis in stems, shoots and roots.

Periderm is further classified into  phelloderm, phellogen and cork as is the case in dicots and gymnosperms.

Phellogen or cork cambium is the meristematic plant tissue that helps in the formation of periderm . Phelloderm is formed in the inner layer of phellogen. The cork (suber) is formed on the outer surface of the phellogen. The suber secretes suberin, an impermeable substance that enters the tissue.