Final answer:
John may use a plastic coverslip to avoid breakage and provide flexibility when crushing the root sample; the iodine stain is used to increase contrast and make cell structures visible.
Step-by-step explanation:
John uses a plastic coverslip instead of a glass one for several reasons. Plastic is less likely to break and cause injury when pressure is applied, it is more flexible which helps in gently crushing the root without damaging the tissues excessively, and it's generally cheaper and easier to handle than glass.
Staining cells is necessary because it increases the contrast between the cell components and the background, making the microscopic structures more visible and easier to identify. The iodine solution John uses is an example of a stain that binds to certain cell components, highlighting features like the nucleus and cell walls in plant cells.