Clayberg, C. D.
During the summer of 1958 at Mt. Carmel, Conn., male-sterile plants (ms\16) of the variety Pritchard (source, Rick) were noticed to be setting fruit almost as well as their fertile sibs. Five fruit each were collected at random from every fertile and male-sterile plant, and the seed was extracted. The seven male-sterile plants had an average seed set of 91 seeds per fruit, while the seven fertile sibs averaged 290-300 seeds per fruit. The observed seed set of the male-sterile plants thus was 30% of their potential total seed set, assuming the latter to equal that of the fertile sibs. This percentage is one measure of the effectiveness of natural cross-pollination. None of the seed set on male-sterile plants is likely the result of selfing, because study of two separate collections, each of three flowers per plant in this family, revealed that none of the male-sterile plants was producing stainable pollen.
Elsewhere in the same field were several small plantings of a potato leafed variety, Geneva 11, surrounded for at least several rows on either side by varieties with normal foliage. Open-pollinated seed was collected from these potato-leafed plants for a second measure of natural cross-pollination effectiveness. Of 1098 seedlings grown only 28 had normal foliage. The effectiveness of outcrossing here was 2.6%.
Two major factors can account for the difference between these two estimates of natural cross-pollination effectiveness. The relative abundance of self-pollen shed by the potato-leafed plant to outcross pollen introduced by the principle insect visitors, bumblebees, gives a great competitive advantage to the self pollen. Secondly, in Geneva 11 the stigma is either level with the tip of the anther cone or slightly inserted, whereas in ms\16 Pritchard it is exserted 1-2 mm, which affords better opportunity for outcrossing than the less exposed stigma.