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\textbf{Text S4}}
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\section*{Robustness}
In Figure S2 some simulation outcomes are shown to visualize the sensitivity of the predicted auxin accumulation pattern for variations of parameters $\theta$ and $d_{dep}$. For $d_{dep}=0$ the pattern does not differ dramatically from that of the nominal parameter set. Increasing $d_{dep}$ to 100 times its nominal value, however, does result in a dramatic pattern change. Without active transport, the uniform auxin concentration equals one, so decreasing $\theta$ from 3 to 2 is substantial. The resulting pattern differs only little from that of nominal simulations. Increasing $\theta$ to 60 shows large undifferentiated (green), but elevated auxin levels in almost the same pattern as in nominal simulations.

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