Example of Measurement Sampling and *k* Value Distributions of Sieve Plates and Secondary Phloem Areas.

**(A)** Overview of parameters measured for calculations on sieve tube–specific conductivity. The pore area (*a*_{p}) was measured to calculate pore radius. Marginal portions of the sieve plate were sometimes covered by overhanging sidewalls. Therefore, the sieve plate area (*a*_{s}) had to be estimated to some extent to calculate plate radius and correct conductivity of the visible area (*a*_{v}). Increase in interporal wall thickness (*w*_{p}) was measured for calculations on callose deposition rates.

**(B)** Pore number, size, and shape of sieve plates may vary considerably even in adjacent tubes.

**(C)** Color-coded overview of the percentage of contribution of individual pores to the total sieve plate conductivity in *Cucurbita*. The larger central pores provide the majority of conductivity. The pattern seems to account for laminar flow.

**(D)** Color-coded sieve plate of *Phaseolus*. A similar but less pronounced pattern as in *Cucurbita* can be found in *Phaseolus*.

**(E)** Color-coded overview of specific conductivity of individual tubes in a vascular bundle of *Cucurbita*. Red, >60 μm^{2}; salmon, 40 to 60 μm^{2}; ocher, 20 to 40 μm^{2}; yellow, <20 μm^{2}.

**(F)** Color-coded overview of specific conductivity of individual tubes in the vasculature of *Phaseolus*. Red, >8 μm^{2}; salmon, 4 to 8 μm^{2}; ocher, 2 to 4 μm^{2}; yellow, <2 μm^{2}.

Bars = 10 μm in **(A)** and **(D)**, 30 μm in **(B)** and **(C)**, 500 μm in **(E)**, and 200 μm in **(F)**.

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