Measurement and analysis of tip growth

Tip growth is basic and deceptively simple. A single cell has a wall that is cylindrical with a prolate spheroid as an end cap. The growth takes place in the end cap. The mechanical loading which drives the growth consists of turgor pressure of magnitude 5-10 atmospheres.We report recent measurements on the geometry of the growth that provide detail of the shape of the end cap. In addition, fluorescent beads were placed at points on the cap, whose positions with time were recorded. A Lagrangian formulation provides the kinematics of the growth process, and indicates that for steady-state, axisymmetric growth, the current velocity of a bead depends on only the geometry and the stretch ratio. The ratio of current mean strain rate to mean stress provides a coefficient of growth rate. This depends on position, which can be interpreted as a dependence on the stretch ratio, i.e., the past history of the wall element. Thus a simple mechanical description appears to be appropriate. However, the stability of the process has not yet been considered.