The consists if the fibers that surround the medulla.

The answer is cortex

I learnt this two days ago 

Answer and explanation;Chemosynthesis is the process in which compounds such as carbon compounds and other molecules such as hydrogen sulfide are used as sources of energy.It involves oxidation of methane or an inorganic compound, such as hydrogen sulfide or hydrogen gas to act as the energy source. Organisms that undertake chemosynthesis are called chemoautotrophs.

Examples of chemoautotrophs include;

Nitrogen fixing bacteria located in the soil,Iron oxidizing bacteria located in the lava beds, and Sulfur oxidizing bacteria located in deep sea thermal vents.