Taller trees tend to have smaller leaves, and Kaare Jensen of Harvard University and Maciej Zwieniecki of the University of California in Davis think that they have worked out why. The reason seems to lie in the circulatory system of trees. Glucose is made in leaves by photosynthesis and despite leaves speeding up the process, it is possible that the stems, branches and trunk act as bottlenecks. At some point it is simply not worth growing larger leaves because the effect becomes more pronounced the taller the tree and the greater these bottlenecks.

Of course, leaves cannot be too small either, and detailed calculations based on data from 1925 species – with leaves from a few millimetres to more than a metre long – then constrain the maximum height of a tree to be about 100 m. The tallest California redwoods reach about 116 m.