Simple Physics May Limit the Size of Leaves - ScienceNOW: The researchers then considered how the total flow of sap and energy varies with leaf length. If the leaves are big, then resistance from the trunk limits the flow. In fact, making the leaves bigger than a certain maximum length yields no additional flow or benefit. On the other hand, if the leaves are very small, their resistance limits the flow. And if a leaf is shorter than a certain minimum length, then the sap would flow through the phloem more slowly than it could simply diffuse through the tree. At that point, the phloem plumbing would become useless.
In fact, these limits neatly fit the observed pattern of leaf sizes, the researchers report. And as tree height increases, the two limits converge and cross at roughly 100 meters: the height of the tallest angiosperms. That means trees taller than 100 meters simply could not produce leaves that obey both length limits, setting a limit for tree height, Jensen says.