Boots and shoes are great for sketching practice and for polishing up on your observational skills. Trees are also great for sketching from life but they can be drawn from your imagination too. We all know what a tree looks like; a thick trunk with branches that spread out from the top of the trunk. Each branch has smaller limbs that spread out from it which may or may not have leaves on them. The point is an imaginative tree does not have to be an accurate representation of any specific tree species. As long as it displays the basic elements of a tree your imagination will do the rest. When it comes to drawing, your imaginative skills need exercising as well as your observational skills.