parastichy

An imaginary spiral around a stem apex joining adjacent leaf primordia. Two parastichies can be visualized running in opposite directions (see diagram). If a plant is described as having, for example, (3 + 5) or 3/5 *phyllotaxis, this means that the shallower of the two parastichies joins every third primordium formed at the apex (1, 4, 7, 10, etc.) and the steeper parastichy joins every fifth primordium (1, 6, 11, 16, etc.).