Which of the following changes occur during differentiation of tracheary elements?
I. Formation of secondary cell wall 
II. Loss of protoplasm
III. lumen formation along its length 
IV. Lignification of cell wall

During differentiation, cells undergo few to major structural changes both in their cell walls and protoplasm. During the formation of  a tracheary element, the cells would lose their protoplasm. They also develop a very strong, elastic, lignocellulosic secondary cell walls, to carry water to long distances even under extreme tension.