Brewbaker, JamesL. (Brookhaven National Laboratory, Upton, New York), and SanatK. Majumder.Cultural studies of the pollen population effect and the self‐incompatibility inhibition.Amer. Jour. Bot. 48(6): 457–464. Illus. 1961.—A significant effect of decreasing population size on pollen germination in vitro was observed in 8 angiosperm genera. Reduction of pollen germination percentages (Y) occurred linearly inPetunia inflata, with decrease of population size (X) below 200 grains per 0.01–ml drop, and differences from linear regression of Y = 1.43 + 0.39X were not significant. Water extracts of pollen and other plant parts contained a factor or factors which could overcome fully the population effect. The pollen growth factor was dialyzable, insoluble in ether, relatively heat‐stable, and was not replaceable by kinetin or auxin. Cultural requirements of petunia pollen were studied in detail and linear growth rates in vitro of 122 μ/hr and in situ of 900 μ/hr were recorded. Growth in hanging drops tapered off in ca. 6 hr, at about the time of second mitosis. Pollen tube inhibition by incompatibility (S) alleles in situ also was observed after about 6 hr. The inhibition of pollen‐tube growth by stylar extracts was investigated by in‐vitro matings of 6 S allele genotypes. Highly significant differences in 5 hr growth were observed in a study of 4200 tubes; incompatible matings averaged 342 μ, semi‐compatible (e.g., S11× S1S2) averaged 434 μ, and compatible averaged 516 μ. In semi‐vitro studies of stylar segments which were pollinated and placed on agar, incompatibility was consistently and clearly evident. It is proposed that the pollen growth factor or factors (“PGF”) may be consumed during growth and that incompatibility inhibits the production or utilization by, or transfer to the pollen tube of PGF. Assuming that second mitosis involves extraordinary demands for PGF, the differences between bi‐nucleate and tri‐nucleate pollen grains in vitro and in incompatibile matings thus might be satisfactorily resolved.