Endocarp
heartwood
Seed
The innermost layer of the hair is the Medulla.
the pistil and stamen
the innermost part of something
The middle.
The innermost part of an object is called its core.
The pistil is the innermost whorl of the flower
Core is defined as "the central or innermost part."
The core.
Because it is the innermost part of the planet.
It's called the lower mantle.
Bone marrow.
There is no layer of Earth that meets that description.