Traditionally, biologists have celebrated the trunk, branch and twig system of a tree as no accident. Many mathematical formulas have suggested it is the best, least wasteful way to design a distribution network. But the very end of such a network, the leaf, has a different architecture. Unlike the xylem and phloem, the veins in a leaf cross-link and loop. Francis Corson of Rockefeller University in New York used computer models to examine why these loops exist.
From an evolutionary point of view, loops seem inefficient because of the redundancy inherent in a looped network. Dr Corson’s models show, however, that this inefficiency is true only if demand for water and the nutrients it contains is constant. By studying fluctuations in demand he discovered one purpose of the loops: they allow for a more nuanced delivery system. Flows can be rerouted through the network in response to local pressures in the environment, such as different evaporation rates in different parts of a leaf.
It’s interesting to think of this configuration vis a vis urban/suburban street networks, when less permeable systems push traffic to larger arterial systems — a benefit for those living in the less permeable areas (say, the second photo above, which I believe comes from a stalled subdivision in Florida), until of course there’s some traffic issue on the main line and less opportunity for rerouting flows. The leaf has no cul-de-sacs, no dead-ends.
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