genetic spiral

(developmental spiral) A hypothetical spiral formed by drawing a line through and joining the centres of successive leaf primordia at a shoot apex. The form of the spiral depends on the angle of divergence and the distance between successive primordia.  The first is often 137.5° and the second depends on the rate of vertical growth of the apex. It is often found that this distance increases geometrically. Compare parastichy. See phyllotaxis.