Conifers are the world’s tallest, widest, and oldest trees. But they have some of the smallest leaves in the plant kingdom, with most never growing beyond 6 centimeters. To find out why, scientists mathematically modeled how liquid nutrients move through the needlelike leaves of conifers, including redwoods and cedars. They focused on sugars, which are produced in the leaves by photosynthesis and transported through tubes 20 times narrower than a human hair to roots and new shoots. But moving fluid through such a narrow space requires a lot of pressure—provided by water-filled cells lining the tube—and there’s an upper limit to how much water can enter those cells. By comparing the pressure necessary to pump sugars with the maximum pressure provided by the cells, the scientists showed that sugars couldn’t be efficiently moved through tubes any longer than about 5 centimeters, they will report in a forthcoming issue of Physical Review E. This length is consistent with data from more than 500 conifer species, with a few exceptions. Species with unexpectedly large leaves, like some pine trees, may have evolved to optimize light capture or gas exchange for photosynthesis, the researchers note. Among trees, the new findings are limited to conifers, whose needlelike leaves help shed snow. But they may extend well beyond the plant kingdom to help researchers understand other processes, like fluid transport in kidneys.