Geotropic Curvatures in Roots of Cress (Lepidium sativum)

Roots of cress growing between two agar slices develop an asymmetry in the extreme root tip region after 10 to 20 min of horizontal stimulation. After prolonged stimulation (exceeding 50 min) the asymmetry disappears and after 3 h the curvature is distributed over the entire growing region. The course of the initial stages in the geotropic curvature has been followed by light microscopy and scanning electron microscopy. — When stimulated at an angle of 135° with the gravitational force, the asymmetry in the root tip is clearly visible after 10 min of stimulation. The asymmetry in the root cap can be explained by a difference in the elongation rate of the epidermal cells on the upper and lower sides of the stimulated root. The disappearance of the asymmetry is followed by a second phase in which there is a differential growth of the cortical cells on the two sides of the elongation zone. The average growth rate of cells in the upper half of the apical region during the first 50 min of continuous stimulation is 1.5 μm per min, while the elongation rate of the entire root is 16.2 μm per min. Only small modifications in the elongation rates were observed when stimulated and unstimulated roots were rotated parallel to the horizontal axis of a klinostat at 2 rpm. The ultimate curvature developed after 50 min is unaffected by stimulation times exceeding the reaction time which for cress roots has been found to be about 5 min. The two phases in the development of geotropic curvature are discussed in view of the statolith theory.