Fig. 4.

Diagram illustrating the movement of a root tip past a position in the soil (solid horizontal line), and how the extension rate of the root tip (V) can be used to calculate the time (t) between the arrival of the tip and the arrival of a location or length (L) along a root axis in the equation: t = L/V. This gives the time that the rhizosphere has been developing at that location. Increasing L corresponds to such events as the root cap passing, reaching the end of the growth zone, and emergence of branch roots. If the root tip stops elongating, t = L/V is not longer useful in estimating the time the rhizosphere has been developing.