What is the term given to the thimble-like structure that covers the root at the apex?
Root cap
The root cap function is to cover and protect the cells at the end of the root of a plant. The root cap is a special layer of tissue which protects the tip of a growing root and helps to anchor the plant. This is a universal feature of angiosperm, gymnosperm, and pteridophyte roots.
The root is covered at the apex by a thimble-like structure and this thimble is a small hard pitted cup like strcuture present in root apex for protection.