POLLEN TUBE GROWTH AND SEED SET IN SELF‐INCOMPATIBLE AND SELF‐COMPATIBLE LEAVENWORTHIA (CRUCIFERAE) POPULATIONS

S ummary Four geographic races of Leavenworthia crassa and one race of L. alabamica are described as self‐incompatible, since the average frequencies of fruit set and of pollen tube growth into the style are significantly lower after self‐pollination than after cross‐pollination. The frequency of successful self‐pollination in individual plants of these races varies from zero to 100%, and is positively correlated with the average number of seeds in self‐pollinated siliquas. Eleven races of L. crassa and three races of L. alabaimca are adapted to self‐pollination and are described as self‐compatible, although self‐pollinations succeed 2% less frequently than cross‐pollinations. Self‐pollinated siliquas of these races contain an average of 12% fewer seeds than cross‐pollinated siliquas; this is probably an expression of cryptic self‐incompatibility and not the result of the exposure of seed lethals on inbreeding. There is, therefore, no absolute distinction between self‐incompatibility and self‐compatibility in Leavemcorthia . The growth of pollen tubes is compared in successful and unsuccessful pollinations of the same type, in self‐ and cross‐pollinations of the same individuals, and in self‐incompatible and self‐compatible races. Pollen grains react with the stigma even in unsuccessful pollinations and some pollen grains are inhibited in successful pollinations. In both self‐incompatible and self‐compatible races, the average number of emptied pollen grains on the stigmata of successful pollinations (an estimate of the number of pollen tubes in the styles) is similar after self‐ and cross‐pollinations. But the average number of emptied pollen grains is lower in self‐incompatible races than in self‐compatible races. The results are interpreted in terms of the response of pollen grains to an essential growth substance supplied by the stigma.