Four differences between bone and cartilage are:

1. Composition: Bones are composed of hard, mineralized tissue, primarily calcium and phosphate salts, while cartilage is composed of a tough, flexible matrix of collagen fibers embedded in a gel-like substance.
2. Structure: Bones have a complex structure with a dense outer layer (cortical bone) and a spongy inner layer (cancellous bone), while cartilage is a homogeneous, flexible tissue.
3. Function: Bones provide structural support, protection of internal organs, and serve as attachment sites for muscles, while cartilage acts as a cushion between bones, reducing friction and absorbing shock.
4. Blood supply: Bones have a rich blood supply, allowing for rapid growth, repair, and remodeling, while cartilage is avascular and receives nutrients through diffusion from surrounding tissues.

The cells of xylem include:

1. Tracheids: Responsible for water transport and provide structural support.
2. Vessel elements: Specialized for water conduction and have perforations called pits for water movement.
3. Fibers: Provide mechanical support and strength to the xylem tissue.
4. Parenchyma cells: Store starch and other substances, and may also have a role in water storage.