The outside of the bone consists of a layer of connective tissue called the periosteum. Additionally, the outer shell of the long bone is compact bone, then a deeper layer of cancellous bone (spongy bone) which contains in the medullary cavity the bone marrow.
They either get their source of nutrients from something else. Or, they could also have shorter roots so that they only reach the top of the soil. They probably won't need as much nutrients to survive either.
