The kinetics of tip growth in the Nitella rhizoid

A detailed observation of the growth kinetics of the rhizoidal cell in Nitella was made possible by application of artificial markers on the cell surface. The elongation of the cell is linear at the rate of 1.7 μm/min (2.5 mm/day), and the activity of the elongation is limited to the cell's apical dome (tip growth) whose shape is unchanged during the elongation. The greatest relative expansion, 0.15 per min (15% per min), occurs at the tip region of the dome where the expansion is roughly isotropic. In the remaining area of the upper half of the dome the expansion is apparently longitudinal stretching (meridional anisotropy), and in the lower half, transverse stretching (latitudinal anisotropy). The observed directionality of the expansion is compared with the expansion of some theoretical cases in which a perfect hemispherical shape and a certain degree of the directionality (described by an allometric coefficient) are maintained during the expansion, and with a similar observation made from the apical cell of the Nitella shoot.