Model of pollen tube growth. (A) Conceptual depiction of the directed and random components of turning. The directed component (black arrow) is calculated based on the gradient of the attractant. The random component is a random angle added to this. The gray shaded regions depict one standard deviation of the Gaussian distribution for the random angle. (B) Dynamics of a model of the gradient. The model gives a theoretical concentration of the attractant (Eq. 3 in Methods), and the gradient is derived from this concentration. Here the magnitude of the gradient from a single ovule, oriented toward the ovule micropyle is shown. Top: Depiction of the model for the attractant gradient as a function of distance from the micropyle. The different curves (top to bottom) are for the gradient after the source has released the attractant for 4.5 hours, 2.5 hours, and 0.5 hours. Bottom: Depiction of the model for the attractant gradient as a function of time on the medium. The different curves (top to bottom) are for distances of 0 μm, 50 μm , 100 μm , and 150 μm from the micropyle.