Science: The vascular system of trees may determine their maximum height and leaf size, according to a recent study published in Physical Review Letters. Kaare Jensen of Harvard University and Maciej Zwieniecki of the University of California, Davis, have observed that leaf size varies more in shorter trees than in taller ones. In their study of angiosperms, which include maples and oaks, they looked at how sap produced in the leaves travels through a network of pipelike cells to the trunk and roots. They determined that a tree’s leaves have to be big enough to get the sap flowing fast enough to overcome the resistance in the trunk. However, once the tree reaches a certain height, its resistance becomes so large that leaf size ceases to matter. Although many have found the theory plausible, others argue that it is too simple and fails to take into account the many variables in such systems.