A tree is anchored in the ground using a network of roots, which spread and grow thicker in proportion to the growth of the tree above the ground. In a mature tree, most of the cells of the trunk, roots, and branches are dead or inactive. All growth of new tissue takes place at only a few points on the tree, by the division of specialized cells. These actively growing areas are located at the tips of branches and roots and in a thin layer just inside the bark. Lastly, trees have reproductive structures; either flowers or cones.
Simple enough, but I say all this to help you find the essential markers to identify a tree. Leaves, bark, twigs and fruit can make quick work of tree identification. Shape, although not a tree "part", plays a key role in tree species characteristics.