Pollen tube behavior is consistent with turning in response to a gradient of an attractant across the tip surface. (A) Schematic of gradient-following model. The pollen tube tip is treated as flat. A gradient in the attractant (G
) concentration gives a difference in concentration Δc between the two sides of the tip. (B) Fit of θ
= Asin θ
+ ε for points 0-50 μm from the micropyle and 4-hour incubation time. In all fits, ε was not significantly different from zero. The slope A can be considered the average response of the pollen tubes to the ovule. (C) Fits were obtained at varying distances from the closest micropyle: 0-50 μm, 50-100 μm, 100-150 μm, and 150-200 μm. The turning response (the slope A) measures the average tendency for pollen tubes to turn toward the micropyle based on the hypothesis that the turns sense a change in the concentration of an attractant across the tip. Turning responses are given for data collected with 0-, 2-, and 4-hour ovule incubation times and also for heat-treated (boiled) ovules. Error bars are the standard errors determined by the linear regression.