Livingston G.K. and Stettler R.F. Radiation-induced stimulation of pollen-tube elongation in Douglas-fir. Radiation Botany 13, 65–72, 1973.-Mature pollen of Douglas-fir (Pseudotsuga menziesii) was subjected to gamma radiation from a 60Co source at exposures of 4, 8, 16, 32, 64, 128 and 256 kR and incubated on an agar medium in vitro. Whereas germination per cents remained constant regardless of treatment, pollen-tube lengths at 48 hr of incubation were positively correlated with radiation dose. The same results were found with X rays. Irradiation seemed to both speed up germination and increase the rate of elongation. Oxygen uptake, as measured by a respirometer, increased proportionately to dose up to 64 kR but declined at higher doses. At the microscopic level, the two highest doses caused an exfoliation of the outer layers of the inline at the time of germination. It is suggested that the observed stimulation of pollen-tube elongation is brought about by an enhanced metabolism at the lower doses; but by a physical change in cell-wall properties at the higher doses.