Trees are what supply the oxygen in the air that keeps us alive. They create oxygen from something that is called photosynthesis. The defence mechanism used by trees against bugs and birds is a sticky sap the flows from the outer layer of hard wood. The roots stretch deep into the dirt in search for water, tearing up and cracking any sidewalks or roads in the way. Roots are a key part in a tree because they do not only supply a tree with its water but they hold a tree firmly in the ground.