Figure 1.

Hypothetical consequences of mechanical interactions between the root tip and gel surface. A, A schematic of the various physical forces suspected to be involved in root waving in the presence of physical interactions between the root tip and gel surface. Normal gravitropic bending could affect the magnitude of root tip impedance. Arrows over the root tip in the right panel indicate right-handed (RH) and left-handed (LH) bending and twisting. The region behind the root tip shown here has negative curvature. B, Follow-the-leader growth. The root tip prescribes the final shape of the root. C, Wagging tail growth. In the absence of friction, the root tip is free to move, and the gel surface acts simply as a two-dimensional constraint on root growth. An oscillatory pattern develops due to a lateral gravitropic over-correction or to circumnutation. D, Loaded beam growth. Surface interactions between the root tip and the gel surface impede the movement of the root tip relative to the expansion of the elongation zone. This amplifies whatever root curvature is already present behind the root tip up to the point of first root hair development. In each case (B, C, and D), φ is the angle between the direction the root tip is pointing (arrow) and the downward slope of the gel surface (dotted line), here shown as an angle of positive magnitude (i.e. to the left in the frame of reference of the root).