Photos Same Pattern, Different Things
Auxin is believed to work by appearing on the surface of leaves and triggering certain cells to transport it out of the leaf through the stem. In a self-reinforcing process, these transporting cells become able to move more auxin and stimulate neighboring cells to do the same. Eventually the surface cells differentiate into veins, creating a network rather like tributaries flowing down to a river. But the problem with this mechanism, Jagla pointed out, is that a river network doesn't form closed loops. With the cell-collapse hypothesis, "veins can form closed paths without any limitation," Jagla said. One thing his model does not explain is how plant species can have different patterns of veins at the largest scale. For instance, in some species, secondary veins branch off the either side of the main vein in an alternating pattern, while others branch off symmetrically. Jagla proposes that genetic factors driven by auxin may determine this big-picture pattern, while the smaller vein patterns form via cell collapse. "We are convinced that the full explanation involves both auxin and elastic stress," he said. How such a two-phase mechanism would come about is difficult to imagine, said Pavel Dimitrov of Yale University in New Haven, Conn. He published a model that explained closed loops in leaf vein networks using an auxin-based mechanism that relies on how auxin concentrations change across the leaf over time. He sees no need for another explanation. Jagla hopes his model will encourage biologists to seek evidence for this hypothesis at the molecular and cellular level within real leaves to help settle the question. "The jury is still out on leaf venation, and it will be out for a long time," Kramer said. Related Links: |
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