E. Katifori/Rockefeller Univ.
Alternate Routes. The veins in a lemon leaf resemble a tree but also contain loops that allow colored die to reach distant parts of the leaf beyond a circular hole (top). A ginkgo leaf has no loops to bypass the hole (bottom). Loops arose to handle such damage and the fluctuations in nutrient needs, according to two reports. A third paper discusses the evolution of blood vessel networks. (See videos below.)