Plants assume a variety of forms, and trees are just one of them. What makes or breaks a tree can come down to some pretty specific characteristics, based on how the plant develops as a result of how it evolved. Trees don’t all comprise one closely related group, like insects or mammals. So, how exactly do trees get classified as trees? Max G. Levy explains what distinguishes these woody plants. Lesson by Max G. Levy, directed by Evandro D'Hipolito.