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\textbf{Text S3}}
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\section*{Model calibration}\label{sec:parestimation}
The simulation model is calibrated to the first developmental stages of an \textit{Arabidopsis} FM by estimating 1) the meristem size that corresponds to auxin inflow at the equator in stage 2, by looking at the timing of the formation of sepal accumulation sites, and 2) the termination of meristematic cell growth in stage 6. The sizes are expressed in numbers of meristem cells, that are defined as model parameters. The rest of the parameters is taken from literature or is optimized (see below).
\subsection*{Cell numbers in stages 2 and 6}
To make a realistic estimation of the cell number, we assume that the true number lies in a range between the number of cells on a flat meristem, and on a spherical meristem.
For a flat meristem, we have an area of $A_m=\pi r^2$ with $r$ the radius. The area of a cell is $A_c=\pi r_c^2$ with $r_c$ the cell radius (typically $2.5 \mu m$). Thus, the number of cells on a flat meristem is $A_m/A_c=(r/r_c)^2$. We estimate from Figure 1E in the main text
that the radius of the meristematic tissue in stage 2/3 is 31 $\mu m$, giving an estimate of 156 stem cells. Since in this stage auxin accumulations have already formed around the equator, we assume that the auxin inflow takes place when the meristem surface is smaller, and consists of 50 cells. This corresponds to a radius of 18 $\mu m$ on a flat meristem. Some time later, the auxin has been transported over the equator, and we assume the corresponding meristem size to be at 100 cells. In stage 6 the estimation of the radius, based on Figure 1A in the main text, is 56 $\mu m$, giving an estimate of 502 stem cells. If we cover a semi-spherical meristem with stem cells that have approximately flat surfaces, we get $A_m=2\pi r_m^2$ and the formula for $A_c$ remains the same. So then the estimated cell number doubles to 1004. Based on this, and the observation that the meristematic tissue size stays more or less constant after stage 6, we select $nc_{end}=700$ cells for growth termination of the meristem.
\subsection*{Parameter values }
In \cite{merks} references are given for the values of $D$, $T$ ($1.5\cdot10^{-5}$ and 0.08), and we use smaller values. Although it was indicated there that these values can have a considerable range, it is the ratio $T/D$ that largely determines the distances between auxin maxima \cite{jonsson}, and this is chosen to be similar to that used in \cite{merks}. We estimate the magnitude of the Michaelis-Menten parameters $k_5$, $k_a$, $k_m$ and $k_r$ to be of the order 1, which is equal to typical values of their corresponding variables $A_i$ and $P_i$. The typical value of $A$ equals 1 since in steady state, without inflow $\phi_b$, and for uniform $A_i$, equation (1) in Text S2 gives $A=\frac{p}{d_{dec}}=1$.
In \cite{jonsson} the value of $\frac{k_2}{k_1}=0.4$ is based on experimental evidence. We chose a similar value of $\frac{k_2}{k_1}=0.3$.
Table S1 shows the resulting nominal parameter values, after optimization of $\gamma, k_4, k_5$, and $\theta$.
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