Question

what is periphery in the case of plants

Answer 1

I think the question means the plant anatomy.
Then we can consider the
Bark as the periphery in the case of wood.

Bark is the outermost layers of stems and roots of woody plants.Plants with bark include trees, woody vines,and shrubs. Bark refers to all the tissues outside the vascular cambium and is a nontechnical term.It overlays the wood and consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost area of the periderm. The outer bark in older stems includes the dead tissue on the surface of the stems, along with parts of the innermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees which lies external to the last formed periderm is also called the rhytidome