Two different genetic systems of incompatibility between pollen and style are known. One, heterogamety, depends upon the genotype of the individual pollen grain; the other, heterostyly, upon the genotype of its parent. We do not know whether the two types are physiologically related. The specificity of heterogamety indicates an immunity reaction. The effect of temperature on pollen-tube growth in the two systems was measured in order to discover their relationship. Compatible pollinations of both systems showed increased rate of growth with increased temperature until the lethal point was approached at about 35 degrees C. Incompatible pollinations of both systems showed an optimum growth rate between 15 and 20 degrees C. The physiological method of inhibition is therefore probably related although its genetic basis is different. The different rate of growth at different temperatures gives different total growth at inhibition, and at the most favourable temperature there may be no inhibition at all. There is therefore no specific inhibitory zone in these plants, although elsewhere the top of the style may provide such a zone. Certain genotypes of Oenothera organensis show such powerful incompatibility that no temperature sensitivity can be discovered. This extreme modification is determined by the pollen parent's genotype, like the main action in heterostyly. In heterostyled plants thrum pollen has to grow down the longer pin style; it is adapted to this in two ways. In Primula it is larger, in Linum grandiflorum it has a higher osmotic pressure. In either case, presumably, it has the higher dry weight. In two heterostyled Primula species thrum pollen grows faster down the long-pin style than pin pollen does down the short-thrum style. But in the illegitimate matings thrum pollen is more strongly inhibited. There is therefore a differentiation of the mechanism adapted to secure equal regularity of cross-fertilization of the two types.